Garry's mod optimization problem?

Hello, I’m working on a big ol’ map project of mine, my next (and greatest) gm_underground type map. When i test it in episode two, everything runs perfectly, no frame rate issues (on full graphics settings), even with fast vis. But for some strange reason, when i go to test my map out in garry’s mod, most of the map lags as if there were no walls or something awful like that.
Has anybody encountered a similar issue, and if so, is there any way to fix this?
Any help at all would be appreciated, thank you for taking the time to read this!
(If it is really necessary, i can post pictures of where it lags the most.)

Process monitor is usually a good tool to figure out why the game is getting crap performance. It helped me find that annoying 8.vmt bug (which still partially exists) that destroys framerates.

Run the map with that running and look for file accesses from hl2.exe. If hl2.exe is spamming requests for some file, it could cause poor framerates (also check with +showbudget)