Why is my iMac running slow--??

Question

Hardware Information:
          iMac (21.5-inch, Late 2012)
          iMac - model: iMac13,1
          1 2.7 GHz Intel Core i5 CPU: 4 cores
          8 GB RAM
Video Information:
          NVIDIA GeForce GT 640M - VRAM: 512 MB
Audio Plug-ins:
          AirPlay: Version: 1.7
          iSightAudio: Version: 7.7.1
System Software:
          OS X 10.8.5 (12F45) - Uptime: 0 days 0:14:5
Disk Information:
          APPLE HDD ST1000LM024 disk0 : (1 TB)
                    disk0s1 (disk0s1) <not mounted>: 209.7 MB
                    Macintosh HD (disk0s2) /: 999.35 GB (544.74 GB free)
                    Recovery HD (disk0s3) <not mounted>: 650 MB
USB Information:
          Seagate Backup+ Desk 3 TB
                    disk5s1 (disk5s1) <not mounted>: 262 KB
                    Seagate Backup Plus Drive (disk5s3) /Volumes/Seagate Backup Plus Drive: 3 TB (1.42 TB free)
          Focusrite A.E. Ltd Impulse
          Apple Inc. FaceTime HD Camera (Built-in)
          Western Digital Ext HDD 1021 2 TB
                    Elements (disk4s1) /Volumes/Elements 1: 2 TB (21.58 GB free)
          Western Digital  External HDD     1 TB
                    Elements (disk3s1) /Volumes/Elements: 1 TB (169.22 GB free)
          Maxtor Corporation Maxtor 3200 320.07 GB
                    BASSLINE Local Disk (disk2s1) /Volumes/BASSLINE Local Disk: 320.07 GB (145.96 GB free)
          Lexmark  2400 Series
          SanDisk Cruzer Edge
          Apple Inc. BRCM20702 Hub
                    Apple Inc. Bluetooth USB Host Controller
FireWire Information:
          Focusrite SAFFIRE_PRO_24 400mbit - 400mbit max
Thunderbolt Information:
          Apple Inc. thunderbolt_bus
                    Apple Inc. Thunderbolt to FireWire Adapter
Kernel Extensions:
          com.seagate.driver.PowSecDriverCore          (5.2.3)
          com.seagate.driver.PowSecLeafDriver_10_5          (5.2.3)
          com.Greatdy.driver.SystemAudioCapture          (1.0.0)
          tc.tctechnologies.driver.PaeFireStudio          (4.0.0
          tc.tctechnologies.driver.Saffire          (4.1.2
          com.Cycling74.driver.Soundflower          (1.6.6)
          com.paceap.kext.pacesupport.snowleopard          (5.9.1)
Problem System Launch Daemons:
          [loaded] com.seagate.TBDecorator.plist
Problem System Launch Agents:
Launch Daemons:
          [loaded] com.adobe.fpsaud.plist
          [loaded] com.microsoft.office.licensing.helper.plist
          [loaded] com.paceap.eden.licensed.plist
          [loaded] PACESupport.plist
          [loaded] tc.tctechnologies.PaeFireStudio.plist
          [loaded] tc.tctechnologies.Saffire.plist
Launch Agents:
          [not loaded] com.adobe.AAM.Updater-1.0.plist
          [loaded] com.nike.nikeplusconnect.plist
User Launch Agents:
          [loaded] com.adobe.AAM.Updater-1.0.plist
          [loaded] com.spotify.webhelper.plist
User Login Items:
          MyDriveConnect
          iTunesHelper
          MyTomTomSA
          Dropbox
          Launch Nikon Message Center 2
          TomTomHOMERunner
          NIHardwareAgent
3rd Party Preference Panes:
          Flash Player
          FUSE for OS X (OSXFUSE)
          Native Instruments USB Audio
Internet Plug-ins:
          AmazonMP3DownloaderPlugin101749.plugin
          Flash Player.plugin
          FlashPlayer-10.6.plugin
          JavaAppletPlugin.plugin
          QuickTime Plugin.plugin
          SharePointBrowserPlugin.plugin
User Internet Plug-ins:
          Aspera Web 3.1.0.67054.plugin
Bad Fonts:
          None
Time Machine:
          Skip System Files: NO
          Mobile backups: OFF
          Auto backup: YES
          Volumes being backed up:
                    Macintosh HD: Disk size: 999.35 GB Disk used: 454.61 GB
          Destinations:
                    Seagate Backup Plus Drive [Local] (Last used)
                    Total size: 3 TB
                    Total number of backups: 81
                    Oldest backup: 2013-02-25 11:58:52 +0000
                    Last backup: 2013-11-07 16:57:40 +0000
                    Size of backup disk: Excellent
                              Backup size 3 TB > (Disk size 999.35 GB X 3)
Top Processes by CPU:
               1%          EtreCheck
               1%          WindowServer
               0%          Activity Monitor
               0%          activitymonitord
               0%          fontd
               0%          MyDriveConnect
               0%          Dropbox
               0%          imagent
Top Processes by Memory:
          180 MB             Safari
          147 MB             WebProcess
          115 MB             Dropbox
          106 MB             mds
          74 MB              Finder
          74 MB              WindowServer
          49 MB              Dock
          41 MB              com.apple.dock.extra
          33 MB              coreservicesd
          25 MB              licenseDaemon
Virtual Memory Statistics:
          5.44 GB            Free RAM
          1.22 GB            Active RAM
          286 MB             Inactive RAM
          1.05 GB            Wired RAM
          321 MB             Page-ins
          0 B                Page-outs

Answer 1

See:
Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.