Massive Lag on Certain Maps

Whenever I load up a map in singleplayer, like rp_city18_v1 or rp_city8/rp_city8_2, it becomes EXTREMELY laggy. To the point it’s two frames per second. My computer is fairly good and can handle GMod fine. I’ve tried googling thing to help, but nothing seems to help. Here’s my system specs.

Operating System: Windows 7 Home Premium 64-Bit
Processor: Intel® Core™ i5-2320 CPU @ 3.00 GHz(4 CPUs), ~3.0GHz
Memory: 6144MB RAM
DirectX Version: DirectX 11
Graphics Card: AMD Radeon HD 7700 Series
Chip Type: ATI display adapter (0x683F)
DAC Type: Internal DAC (400MHz)
Approx. Total Memory: 3812 MB
Current Display Mode: 1360 x 768 (32 bit) (60Hz)

And by the way, the maps work perfectly, no lag at ALL when in multiplayer. So it’s something in singleplayer.

Thanks in advance!

Did you consider checking out your addons?

I also experience a lot of lag on certain maps, even when I’m on video settings lower than what it says is recommended for my computer.

Move your addons folder out of garrysmod.
Set your video settings to the reccomended settings.
Load up the map again, and tell me how it goes then.

Also try the same with a map like gm_flatgrass.

I did gm_flatgrass, it works perfectly. Also, I had done that. I put it on the recommended settings, which is actually maximum, lowered it a bit, it didn’t work for the other maps I listed.