Here is also some stuff that might help, add the following to your \steamapps\common\GarrysMod\garrysmod\cfg\autoexec.cfg file
Code:
gmod_mcore_test 1
mat_queue_mode -1
cl_threaded_bone_setup 1
cl_cmdrate 0
This should help a lot with FPS, it might also not help at all or make things worse, if it does, remove these lines again.
The first 3 lines will make the multi-core support work, the tick-box in game does nothing.
The last line may or may not help with ping, if it makes things worse remove it.