If your computer is overheating, I recommened checking for dust in your computer. My computer side is open due to it overheating far too much. It has helped a lot part from now it generates much more dust requiring for me to clean it about ever 1/2 a month.
I recommened having a parent to have a look at your computer for dust and see if you are able to clean it with some compressed air. If you are scared to break your computer but have noticed that it is dusty, I recommened taking it to a PC specialist for a clean.
You can try restarting your computer and doing as Sours said above. If that fails to work, then I'm afraid this will need to be closed as there isn't much we can do for you. If you want to try a solution from me though, attempt to uninstall GMOD from your hard drive then reinstall on to your SSD, but dsome quick notes:
Slow system performance can derive from a variety of things. If you haven't already, I'd suggest doing various virus scans with software such as
MalwareBytes and other reputable virus/malware cleaning solutions. If all else fails to speed up your system, then I'd recommend looking into a windows reinstall.
If that does nothing to your performance, I'd suggest looking into new hardware.
I had an issue near to it once , try to delete addons that blocking your connection and make the game freeze, verify game cache, try to join.
Tell us if problem fixed or no.
I cannot delete any more addons as I use them
hmm. Try to reinstall them and verify game cache.
I suggest you use this commands before you join the server, it may utilize your quad core better and improve on the lag spikes.
Commands:
gmod_mcore_test 1
mat_queue_mode -1
cl_threaded_bone_setup 1