Due to the size and visual fidelity, the map pushes the boundaries of what the game engine can handle; mainly due to the large amount of assets and the openness of the world. Source is designed for the close-quarters and linear map design of most Half-Life, Counter Strike and Team Fortress maps. Having what is essentially a huge open box cripples the performance pretty fast, mainly due to the archaic way the engine handles visibility testing...
To remedy this, numerous measures have been put in place to try to get the framerate as stable and high as possible.
Technical explanation of the issue[www.mapcore.org]


However; numerous might still not be enough for lower-end or older systems. I have tried to cater towards the most popular hardware according to the steam hardware survey, and seen similar if not better performance compared to maps of similar fidelity and size.


Some at-home remedies you can try if youre having performance issues:

1. Open the console and type This will enable multi-core rendering

2. Open the console and type This will disable water reflections and net you anywhere from 10-50FPS extra! (this works best if you type the command while underwater and looking at the sea floor(it sounds weird but trust me!))


Also check out these:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=718934470
Making Garry's Mod run faster with Source shenanigans [www.dinklebergsgmod.com]