Go into setup on Far Cry Primal after you move it to the E drive and reset it to defaults. Then retune it to the video settings. etc., that you like. Sometimes games remember what drive they are on and store profiles, etc and a simple copy is not sufficient -- you lose the profile. Google to make sure that there is nothing you need to do for the game you are running to get it to use the E disk vs C or D. Make sure you are running with the video setting you think you are using.
The C, D, E drives are all on the same physical drive, so they should give you the same performance unless there are physical errors on the drive. Let's rule this out using windows built in sector scan tool to check for (and reconfigure around) bad sectors.
To check for bad sectors follow this post from Microsoft and make sure you select the Scan for and attempt recovery of bad sectors check box.
https://support.microsoft.com/en-us/help/2641432/check-your-hard-disk-for-errors-in-windows-7 This will take hours so do overnight.
Then force a defrag of your E: using the same properties dialog that you used to scan for errors.
After you have done above, if games are still slow, then do the following:
1. Open windows resource monitor. many ways to do this, here are two: type resmon.exe into the Start Menu search box and press Enter. Or, open the Start Menu and go to "All Programs -> Accessories -> System Tools -> Resource Monitor".
2. Goto the disk tab in resource monitor
3. Goto the disk activity section, note that it shows you the files being accessed then look to the far right and find the "response time" column for each file.
4. Leaving resource monitor running, start your game.
5. When the game lags, use the windows key to get to desktop where you can see Resrouce Monitor. Note the disk activity rate and the response time for the files associated with your game (they will be in the game directory). If the Disk activity is low and the response times are not in the 100s then disk is not slowing your game.
6. You can explore other tabs in Resource monitor to see if CPU spiked to 100% or if network got bad, etc.
Finally, try a bunch of games on E, see if they are all bad or only one. It is very rare that speed of disk drive makes a gaming performance difference, much less that moving a game from one partition to another on the same drive changes anything. Games are video, CPU and network sensitive. The disk is not used at all during gameplay, so really CANNOT directly drop the frame rate from 60 to 20 (I believe the framerate is dropping, but it will be an indirect reason, not because the disk is slow).
OH: final thought: windows does some neat preloading of memory if you have enough memory. It is called superfetch. It learns what games you play and preloads them so they start fast. When you switched drive letters it'll take superfetch a few days to learn your new file access pattern. That shouldn't make enough of a difference to drop framerates, but will slow load times.