IMac CPU3.06 GHz Intel Core 2 Duo, 8GB, Extremely slow and repeatedly "out of Memory" since migration from Mountain Lion to Mavericks

Question

Dear Apple Support Community.
As mentioned by a number of users reporting the same problem, my 2009 iMac CPU3.06 GHz Intel Core 2 Duo, 8GB RAM, has become extremely slow and repeatedly "out of Memory" since it was migrated from Mountain Lion to Mavericks (OS 10.9 then OS 10.9.1).
I have already run CleanMyMac, deleted 5GB of useless stuffs, which did not translate into any improvement. Following the advices collected in other Apple Support Community threads, I have applied EtreCheck the outcome of which is provided below.
For information, I have also upgraded my 2011 MacBook CPU2.3 GHz Core i7, 8GB which became only a bit slower than before, sometimes freezing which it almost never did before. However, I am used to the fact that Mac get usually slower and more prompt fo freeze after upgrading to a more recent OS.
Both my 2009 iMac and my 2011 MacBook have F-Secure installed. So the problem with the 2009 IMac is unlikly to be due to the presence of F-Secure. Interestingly, the few abnormalities found by Etrecheck were exacly identical before I ran CleanMyMac. There are two more findings (mentioned below) since I ran CleanMyMac on the 2009 iMac, which again didn't improve the problem.
So please find below the details of the EtreCheck experience and some additional measurements made with iStat while running EtreCheck:
Hardware Information:
iMac (24-inch, Early 2009)
iMac - model: iMac9,1
1 3.06 GHz Intel Core 2 Duo CPU: 2 cores
8 GB RAM
Video Information:
ATI Radeon HD 4850 - VRAM: 512 MB
Audio Plug-ins:
BluetoothAudioPlugIn: Version: 1.0 - SDK 10.9
AirPlay: Version: 1.9 - SDK 10.9
AppleAVBAudio: Version: 2.0.0 - SDK 10.9
iSightAudio: Version: 7.7.3 - SDK 10.9
System Software:
OS X 10.9.1 (13B42) - Uptime: 0 days 2:40:57
Disk Information:
WDC WD1001FALS-40K1B0 disk0 : (1 TB)
EFI (disk0s1) <not mounted>: 209.7 MB
XooS5 HD (disk0s2) /: 999.35 GB (189.85 GB free)
Recovery HD (disk0s3) <not mounted>: 650 MB
OPTIARC DVD RW AD-5670S 
USB Information:
Apple Inc. Built-in iSight
Canon CanoScan
Apple Computer, Inc. IR Receiver
Apple Inc. BRCM2046 Hub
Apple Inc. Bluetooth USB Host Controller
FireWire Information:
WD My Passport 071D 800mbit - 800mbit max
EFI (disk1s1) <not mounted>: 209.7 MB
disk1s2 (disk1s2) <not mounted>: 2 TB
Boot OS X (disk1s3) <not mounted>: 134.2 MB
Thunderbolt Information:
Kernel Extensions:
com.f-secure.kext.fsauth          (1.0.0d1)
Problem System Launch Daemons:
[failed] com.apple.wdhelper.plist [REMARK: this abnormality appeared only after running CleanMyMac. It does not appear on my 2011 MacBook]
Problem System Launch Agents:
Launch Daemons:
[loaded] com.adobe.fpsaud.plist
[invalid] com.bjango.istatlocaldaemon.plist [REMARK: this abnormality pre-existed to running CleanMyMac. It appears also on my 2011 MacBook]
[loaded] com.f-secure.fsavd.dbhelper.plist
[loaded] com.f-secure.fsavd.plist
[loaded] com.f-secure.fsmac.firewall.plist
[loaded] com.f-secure.fsmac.fsupdated_guts2.plist
[loaded] com.f-secure.fsmac.licensetool.plist
[loaded] com.macpaw.CleanMyMac2.Agent.plist
Launch Agents:
[loaded] com.babylon.activation.plist
[loaded] com.bjango.istatlocal.plist
[loaded] com.f-secure.relauncher.plist
[loaded] com.f-secure.trasher.plist
User Launch Agents:
[loaded] com.adobe.ARM.[...].plist
[loaded] com.google.keystone.agent.plist
[loaded] com.macpaw.CleanMyMac2Helper.diskSpaceWatcher.plist
[loaded] com.macpaw.CleanMyMac2Helper.scheduledScan.plist
[loaded] com.macpaw.CleanMyMac2Helper.trashWatcher.plist
User Login Items:
iTunesHelper
Dropbox
F-Secure Anti-Virus for MAC
3rd Party Preference Panes:
Flash Player
Internet Plug-ins::
FlashPlayer-10.6: Version: 11.9.900.170 - SDK 10.6
Default Browser: Version: 537 - SDK 10.9
AdobePDFViewerNPAPI: Version: 11.0.04 - SDK 10.6
AdobePDFViewer: Version: 11.0.04 - SDK 10.6
DivXBrowserPlugin: Version: 2.1
Flash Player: Version: 11.9.900.170 - SDK 10.6
OfficeLiveBrowserPlugin: Version: 12.2.8
Google Earth Web Plug-in: Version: 6.1
QuickTime Plugin: Version: 7.7.3
iPhotoPhotocast: Version: 7.0
User Internet Plug-ins::
Picasa: Version: 1.0
Bad Fonts:
None
Old applications:
None
Time Machine:
Skip System Files: NO
Mobile backups: OFF
Auto backup: YES
Volumes being backed up:
XooS5 HD: Disk size: 930.71 GB Disk used: 753.90 GB
Destinations:
XooS5 TM [Local] (Last used)
Total size: 2 
Total number of backups: 32
Oldest backup: 2013-09-15 01:11:39 +0000
Last backup: 2013-12-07 12:48:45 +0000
Size of backup disk: Too small. Backup size 2  < (Disk used 753.90 GB X 3)
Time Machine details may not be accurate. All volumes being backed up may not be listed.
REMARK: The above finding is unlikely to be accurate. In fact, my Time Machine is supported by a 2TB WD My Passport Studio external HD connected with a Firewire 800. As mentionned, only 754GB of the 2000GB are used.
Top Processes by CPU:
5%          backupd
5%          Mail
5%          WindowServer
3%          Finder
3%          Dropbox
Top Processes by Memory:
1.60 GB          mds_stores REMARK: this item was marked in red in the outcome of EtreCheck, so I assume there is something wrong there
82 MB          Finder
74 MB          Mail
49 MB          softwareupdated
49 MB          Dropbox
Virtual Memory Statistics:
97 MB          Free RAM
1.21 GB          Active RAM
1.19 GB          Inactive RAM
1.21 GB          Wired RAM
1.83 GB          Page-ins
36 MB          Page-outs
Please find now some additional measurements collected with iStat while running EtreCheck:
Practically:
The iMac can work slowly with only one software e.g. either Mail or Safari or Firefox, but still the below "out of Memory" message occurs about every 3 minutes.
Aperture has becomre to slow to be useable
If trying to write a mail (with Mail) while Safari or Firefox is open, then the "out of Memory" message occurs about every 20 seconds so I need to check on "Resume" several times per minute.
Conclusions
From the above, unless anyone in the Apple Support Community proves to be able to identify a specific problem leading to a way to find a cure, I tend to conclude that Mavericks is not suitable for computers operating with a Core 2 Duo CPU even with a 8GB of RAM.
Unless I missed something, I did not see any warning from Apple Computer Inc. informing their customers they should refrain from upgrading Core 2 Duo Computers.
The only solution I see so far is to downgrade from Mavericks to Mountain Lion. And the Apple Support Community would confirm that it is the only solution, I would appreciate to get your guidance on how to proceed.
Intended action plan
Migrate the above 2009 iMac to a 2014 Mac Mini Server operating on Mavericks to ensure that no data may be lost while downgrading to Montain Lion.
Provided the above downgrading operation is successful, assign this 2009 iMac to partial retirement e.g. Mail, Skype, iTunes  and DVD player.
In advance, I would like to thank the Apple Community for helping me either i) finding a cure to the problem, ii) or confirming that Mavericks cannot operate properly on Core 2 Duo computers, iii) and in such a case, guiding me on how to downgrade to Mountain Lion.
Yours sincerely
Core

Answer 1

... what anti-virus would you recommend using?
Use what is already included with OS X. It already includes everything it needs to protect itself from viruses and malware. Keep it that way with software updates from Apple.
A much better question is "how should I protect my Mac":
Never install any product that claims to "speed up", "clean up", "optimize", or "accelerate" your Mac. Without exception, they will do the opposite.
Never install pirated or "cracked" software, software obtained from dubious websites, or other questionable sources. Illegally obtained software is almost certain to contain malware.
Don’t supply your password in response to a popup window requesting it, unless you know what it is and the reason your credentials are required.
Don’t open email attachments from email addresses that you do not recognize, or click links contained in an email:
Most of these are scams that direct you to fraudulent sites that attempt to convince you to disclose personal information.
Such "phishing" attempts are the 21st century equivalent of a social exploit that has existed since the dawn of civilization. Don’t fall for it.
Apple will never ask you to reveal personal information in an email. If you receive an unexpected email from Apple saying your account will be closed unless you take immediate action, just ignore it. If your iTunes or App Store account becomes disabled for valid reasons, you will know when you try to buy something or log in to this support site, and are unable to.
Don’t install browser extensions unless you understand their purpose. Go to the Safari menu > Preferences > Extensions. If you see any extensions that you do not recognize or understand, simply click the Uninstall button and they will be gone.
Don’t install Java unless you are certain that you need it:
Java, a non-Apple product, is a potential vector for malware. If you are required to use Java, be mindful of that possibility.
Disable Java in Safari > Preferences > Security.
Despite its name JavaScript is unrelated to Java. No malware can infect your Mac through JavaScript. It’s OK to leave it enabled.
Block browser popups: Safari menu > Preferences > Security > and check "Block popup windows":
Popup windows are useful and required for some websites, but popups have devolved to become a common means to deliver targeted advertising that you probably do not want.
Popups themselves cannot infect your Mac, but many contain resource-hungry code that will slow down Internet browsing.
If you ever see a popup indicating it detected registry errors, that your Mac is infected with some ick, or that you won some prize, it is 100% fraudulent. Ignore it.
Ignore hyperventilating popular media outlets that thrive by promoting fear and discord with entertainment products arrogantly presented as "news". Learn what real threats actually exist and how to arm yourself against them:
The most serious threat to your data security is phishing. To date, most of these attempts have been pathetic and are easily recognized, but that is likely to change in the future as criminals become more clever.
OS X viruses do not exist, but intentionally malicious or poorly written code, created by either nefarious or inept individuals, is nothing new.
Never install something without first knowing what it is, what it does, how it works, and how to get rid of it when you don’t want it any more.
If you elect to use "anti-virus" software, familiarize yourself with its limitations and potential to cause adverse effects, and apply the principle immediately preceding this one.
Most such utilities will only slow down and destabilize your Mac while they look for viruses that do not exist, conveying no benefit whatsoever - other than to make you "feel good" about security, when you should actually be exercising sound judgment, derived from accurate knowledge, based on verifiable facts.
Do install updates from Apple as they become available. No one knows more about Macs and how to protect them than the company that builds them.
Summary: Use common sense and caution when you use your Mac, just like you would in any social context. There is no product, utility, or magic talisman that can protect you from all the evils of mankind.