Your motherboard does not have the FX8320 listed as a supported CPU: http://www.asrock.com/mb/NVIDIA/N68-VS3 FX/?cat=CPU
Also, your motherboard does not have any Windows 10 drivers listed for download, so it could be something on that front.
However, before you start swapping hardware and software, you should try a different game that is not based on the Source engine. The Source engine, especially the build CSGO uses, has had numerous issues with performance.
If other games don't give you issues then it's probably CSGO's fault, but if other games do have similar issues then use MSI Afterburner and Task Manager and see if your GPU Core Clock and CPU Speed(GHz) are being boosted to their max(or at least close to it) when gaming.
I know some people have had issues with "power saving" features that would block the GPU/CPU from being utilized properly. AMD has a power saving feature called "ULPS" for their GPUs that can cause this, which can be disabled via MSI Afterburner or Regedit(I would use MSI Afterburner though, just in case).
If that doesn't solve your issue you could try installing Windows 7 since your motherboard supports it and see if it's a compatibility issue with Windows 10. If that doesn't work then it might just be your motherboard then.
Honestly I have a i7-3770 and a R9 390 and I have terrible drops in CSGO and I've tried everything I could think of and nothing has seemed to fix it. I started having issues back when Valve introduced the R8 Revolver, so back in November 2015.
Best of luck!