I had same problem but if you go to singleplayer then go to your desktop, and go to your jobslist and right click HL2.exe (the processer for Gmod) and chose Priorty and put it on high that should help, it helped me!

I had same problem but if you go to singleplayer then go to your desktop, and go to your jobslist and right click HL2.exe (the processer for Gmod) and chose Priorty and put it on high that should help, it helped me!