CSGO is heavily dependent on your CPU, so the first thing that comes to mind is your CPU is getting too hot and starting to throttle. When you run CSGO, monitor your system and see if anything, be it CPU, GPU, motherboard, HDD; anything if it's running too hot.

I would also consider running a check for Malware or viruses as these can slow down a PC.

Alternatively, CSGO caps the menu to 115-119 FPS. Not sure if that's where you're seeing the FPS at, but if you put these into the console, it will uncap your FPS so you can see that way.

fps_max_menu 999
fps_max 999

FPS max menu is only for the menu, FPS max is for when you're in a server.

Are there any other games that are running slow too?

Edit: One other thing that caught my eye is your mention of you trying to optimise games. I'm not sure if AMD ha a software that works similar to Nvidia's GeForce Experience; I would assume Gaming Evolved would be what I'm talking about. Nvidia has a tab for recommended optimisation, which at times can force your settings to go higher than they can run smoothly or will try to upscale your resolution to what THEY think your GPU can run. If AMD has a tab that will automatically adjust your optimisation to what they recommend, then check to see if it's forcing your resolution to say 1440p or anything higher than your native resolution. I would honestly just turn off any recommended optimisation setting if there is one for AMD and leave it all to your own will.