I've found so far that this fix has worked for me as during large amounts of gunfire my fps would drop from 300fps to 11fps.
- Open console with the ` key.
- Type into console "fps_max 60" (ignore "", you may also change the number value).
- Hit enter and play as normal.
This fix essentially limits the fps to not go below 60 (as far as i know). Please try this fix if you have this problem and post back if it does or doesn't work. Other methods of fixing this problem are also welcome to be posted below.
Crazy Swedes Fix.
1.Open your steam library.
2.Right-click on Counter-Strike:Global Offensive
3.Properties
4.General
5.Set launch options...
6.Copy and paste the following in and replace X in -CpuCount=X with how many cores you have on your CPU.
-processheap -high -novid -CpuCount=8
8.Press OK
9.Close
Next time you launch CS:GO, it should be fixed. I know it worked for me at least.(2 confirmed case of crazy swede and agent_wesker).
NOTE: Crazy swedes method using the -high and -CpuCount=8 dedicates a lot of system resources to the game, if your system is not able to cope with these settings it may CRASH (incidents of using these methods have caused crashes on other games).