1. set the cpu and gpu to performance mode and not on energy saving (control panel and nVidia control panel)
2. check for unnecessary background apps and/or services. Disable them from autostart, if necessary.
3. check for viruses, etc.
4. for online games like CS:GO, your fps is also somehow influenced by your connection latency. (if the problem is on the game server, you can not do anything)
5. check also directly on CS:GO forum for other ideas.