I suggest using a mod called Freeze All

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=130550399

If you start lagging. Just type !fr in chat. You can also have a Wiremod E2 run concmd every few minutes.

IF YOU DONT WANT TO FREEZE ALL. YOU CAN ALSO MODIFY YOUR GAME FILES A BIT

There is a Portal Mod called VPhysics Jolt. It also works well with GMOD since their the same engine! It is a recoded version of the Source Physics. It really does improve the performance a big ol' 92%.

https://git.froggi.es/joshua/vphysics_jolt_gmod_builds

Backup the Win64 file inside of your bin of Garry's Mod. Then drag the Win64 from the froggie bin folder to your GMOD (replace all files)