MacBook Pro - Very slow, Full RAM, Fan is constantly on - With EtreCheck, etc.

Question

Hello everyone! Thank you all in advance for any type of help or advice, I appreciate it a lot.
I am using:
MacBook Pro (15-inch, Mid 2012)
Running OS X Yosemite Version 10.10.2
Processor 2,3 GHz Intel Core i7
Memory 4 GB 1600 MHz DDR3
Graphics NVIDIA GeForce GT 650M 512 MB
Macintosh HD has 294,08 GB free (out of 499,25 GB)
I am a student, using my MacBook constantly during the day. Applications that I use daily - Safari, Word, Outlook and Finder are always on; a lot of the time I am also using Photoshop CS6, Skype.
Ever since the update to OS X Yosemite my MacBook did slow down noticeably. I left this issue unattended as I was using uni computer at the time. Recently I returned to the actual studies and design work on my MacBook. I couldn't cope with how slow it was, ended up in forums looking for the solution. I tried a couple of things already (was restarting/shutting down computer in-between these tasks):
1. Disabled FileVault (it was on for moths, I had no idea of what it is), completed the decryption process (it took around 10 hours).
2. After that I ran EtreCheck, that found a couple of actual and potential issues, I more or less sort it out, will attach the new report in the end of the post.
3. Tried to disable Spotlight to affect mds, not sure it worked.
4. Moved to Trash apps I wasn't using, properly uninstalled only a couple of them. Emptied trash.
5. Erased Safari history that emptied about 500MB of RAM, but then it went back to almost 3.98GB full out of 4GB.
At the moment I only have Safari on and kernel_task takes 570MB of memory, used RAM is 3,88GB out of 4GB.
Fan is constantly on, MacBook is very slow. If I understand correctly RAM is the main issue, unfortunately being a student I am not able to buy and install more memory. I also understand that Photoshop takes a lot of RAM memory, what can I do about it? Photoshop is a crucial app for me.
Is there a way to free some of the RAM memory and make my Macbook faster? Would a clean Yosemite install help? (Can Yosemite at all perform properly on a MacBook with only 4GB memory? What will help it run faster?)
Here's my EtreCheck report:
Hardware Information: ℹ️
    MacBook Pro (15-inch, Mid 2012)
    MacBook Pro - model: MacBookPro9,1
    1 2.3 GHz Intel Core i7 CPU: 4-core
    4 GB RAM Upgradeable
        BANK 0/DIMM0
            2 GB DDR3 1600 MHz ok
        BANK 1/DIMM0
            2 GB DDR3 1600 MHz ok
    Bluetooth: Good - Handoff/Airdrop2 supported
    Wireless:  en1: 802.11 a/b/g/n
    Battery Health: Normal - Cycle count 675
Video Information: ℹ️
    Intel HD Graphics 4000
    NVIDIA GeForce GT 650M - VRAM: 512 MB
        Color LCD 1440 x 900
System Software: ℹ️
    OS X 10.10.2 (14C1514) - Time since boot: 1:13:28
Disk Information: ℹ️
    APPLE HDD TOSHIBA MK5065GSXF disk0 : (500,11 GB)
        EFI (disk0s1) <not mounted> : 210 MB
        Macintosh HD (disk0s2) / : 499.25 GB (294.08 GB free)
        Recovery HD (disk0s3) <not mounted>  [Recovery]: 650 MB
    HL-DT-ST DVDRW  GS31N
USB Information: ℹ️
    Apple Inc. FaceTime HD Camera (Built-in)
    Apple Computer, Inc. IR Receiver
    Apple Inc. BRCM20702 Hub
        Apple Inc. Bluetooth USB Host Controller
    Apple Inc. Apple Internal Keyboard / Trackpad
Thunderbolt Information: ℹ️
    Apple Inc. thunderbolt_bus
Gatekeeper: ℹ️
    Mac App Store and identified developers
Launch Agents: ℹ️
    [not loaded]    com.adobe.AAM.Updater-1.0.plist
Launch Daemons: ℹ️
    [loaded]    com.adobe.fpsaud.plist
    [loaded]    com.adobe.SwitchBoard.plist
    [loaded]    com.microsoft.office.licensing.helper.plist
User Launch Agents: ℹ️
    [failed]    com.adobe.ARM.[...].plist
    [loaded]    com.spotify.webhelper.plist
User Login Items: ℹ️
    Google Drive    UNKNOWN  (missing value)  What do I do about these?
    Dropbox    UNKNOWN  (missing value)
    CrossOver CD Helper    UNKNOWN  (missing value)
Internet Plug-ins: ℹ️
    AdobeAAMDetect: Version: AdobeAAMDetect 1.0.0.0 - SDK 10.6 Too many?
    FlashPlayer-10.6: Version: 17.0.0.134 - SDK 10.6
    Default Browser: Version: 600 - SDK 10.10
    Flash Player: Version: 17.0.0.134 - SDK 10.6
    QuickTime Plugin: Version: 7.7.3
    o1dbrowserplugin: Version: 5.40.2.0 - SDK 10.8
    SharePointBrowserPlugin: Version: 14.4.7 - SDK 10.6
    googletalkbrowserplugin: Version: 5.40.2.0 - SDK 10.8
    Silverlight: Version: 5.1.20913.0 - SDK 10.6
    JavaAppletPlugin: Version: 15.0.0 - SDK 10.10
3rd Party Preference Panes: ℹ️
    Flash Player 
    Perian Plan to uninstall
Time Machine: ℹ️
    Time Machine not configured! Is it needed?
Top Processes by CPU: ℹ️
         8%    mds
         3%    WindowServer
         1%    Safari
         0%    fontd
         0%    com.apple.WebKit.Networking
Top Processes by Memory: ℹ️
    185 MB    mds_stores
    142 MB    Safari
    107 MB    Finder
    90 MB    com.apple.WebKit.WebContent
    86 MB    WindowServer
Virtual Memory Information: ℹ️
    733 MB    Free RAM
    1.77 GB    Active RAM
    1.01 GB    Inactive RAM
    777 MB    Wired RAM
    2.12 GB    Page-ins
    12 KB    Page-outs
Diagnostics Information: ℹ️
    Apr 4, 2015, 07:55:31 PM    Self test - passed
    Apr 4, 2015, 01:48:14 PM    /Users/[redacted]/Library/Logs/DiagnosticReports/Skype_2015-04-04-134814_[redac ted].crash
    Apr 3, 2015, 09:24:39 PM    /Library/Logs/DiagnosticReports/Adobe Photoshop CS6_2015-04-03-212439_[redacted].cpu_resource.diag
    Apr 3, 2015, 09:16:37 PM    /Users/[redacted]/Library/Logs/DiagnosticReports/Adobe Photoshop CS6_2015-04-03-211637_[redacted].crash
    Apr 4, 2015, 12:15:29 AM    /Library/Logs/DiagnosticReports/LegacyFileVaultMessageTracer_2015-04-04-001529_ [redacted].crash
Thanks a lot in advance! Looking forward for the possible solutions!
Anna

Answer 1

Let Spotlight finish indexing your HDD.  Stopping it is pointless.
Regarding the fans, it is always running when you turn on the MBP.  It is a question of how fast. If there are light loads on the CPU, the fans should be near silent.  If otherwise, try a SMC reset.
http://support.apple.com/en-us/HT201295
If that does not slow the speed at very light loads then there are other reasons that need to be explored.
If you cannot instal;l additional RAM (your MBP will accept up to 16GB) then you will have to accept slower performance.  Yosemite is a RAM glutton.  The best sources of Mac compatible RAM are Owc and Crucial.
Time Machine or any other backup is only necessary if the data you have on your MBP is important.  Ask yourself this question:  What will be the degree of inconvenience should the internal HDD fail?  All hard drives eventually fail.
Ciao.