Activity Monitor/RAM/Slowness

Question

I updated to Mavericks the day of the update, and I switched to Safari (from Firefox) immediately too (Safari now my default and Firefox having various other tabs open simultaneously).
For the first time ever, my Mac (2011 iMac) gets really slow - hangs, spinning wheel, unresponsiveness. This is often having one of the browsers open too long, with Safari in particular being a massive memory hog, and restarting them fixes it. But poking around activity monitor I've seen some peculiarities.
For instance, my used memory keeps going up, with nothing happening. This morning the computer was using 11.99gb of my 12gb memory, with swap increasing too. So I rebooted, telling it to NOT reopen windows on bootup. With nothing running, ram usage was 3gb, then 4gb, climbing continuously and it's now over 5 with just Firefox running to type this request. App memory is 1.75 and file cache 2.35, which from looking at other threads seem to be pretty high - this is less than 5 minutes after a total restart by the way.
Prior to rebooting I had a few 'not responding' items in AM, soagent and QTKit web content for Safari, no idea if they have any impact on the slowing down (their memory usage was tiny and the computer generally ran fine with them flagged red) but the reboot has got rid of them.
Here's a screenshot of AM taken as soon as the computer restarted, with Firefox the only open program.

Answer 1

I'm afraid Steve is right, your memory is fine.
The symptoms you describe are not memory related (at least that isn't shown in those screenshots).
Here is a great article that shows how the 10.9 memory system has been changed…
http://arstechnica.com/apple/2013/10/os-x-10-9/17/#compressed-memory
See the 2 graphs, only the second one is showing signs that are bad - the thin red edge is indicating the increase in 'Swap used'.
You can witness this for yourself via a built in Apple commands…
Reboot to see the system in it's default state.
Open Activity Monitor & Terminal from /Applications/Utilities.
Select the Memory tab in Activity Monitor
In Terminal enter the following command
memory_pressure -l critical
# note that is a lowercase L
RAM usage will climb, compression will begin the VM will become way more than the system has installed.
Eventually the system will start swapping  (look for RED) - Watch the 'memory pressure' & 'Swap used' as this happens.
In Terminal & hit ctrl+c to stop the process.
Watch the VM & memory pressure return to normal levels.
I'm afraid you will need to look at what is using CPU or disk IO (or both) to see what is causing your issues.
Also don't forget to test in Safe mode (hold shift after the chime until the spinning 'cog' appears). Ensure the login window says 'safe mode'.
https://support.apple.com/kb/HT1455
It will disable all third party extensions & startup items.
If the Mac is better in safe mode it is time to get a system report and look at the third party items. Be aware that some features will be disabled like wifi on some models, graphics drivers will be in a reduced mode - this is normal in safe mode.
Reboot to go back to normal.
Use EtreCheck to see what is running in a normal boot & post it here if you want to look at the running software.
http://www.etresoft.com/etrecheck