I am also a cs-go player, my laptop also starts dropping frames if I play for 3-4 hours, and it goes back to normal after it cools down. Your device having hardware from 3 years ago may be the reason for it to drop frames. Maybe there was an update to the game recently that made it become a game that will doesn't support nicely.
Can you put your device in a cooler room, or near a fan to see if the same issue starts happening at a later time?
If you have all the latest versions of software than there is a big chance it is a hardware issue.
Let me know how it goes.
Mustafa