What’s your OS? Because it has been shown over and over and over again that Windows 7 hates Source and/or vice-versa.
You have three cores, right? Start GMod, Ctrl+Alt+Delete, Processes, right-click on hl2.exe, Set Affinity…, and set the affinity to only one core. You will, unfortunately, have to do this every time you start GMod. There are programs out there that can do this automatically, but they are not often free.
If you STILL get poor framerate, and even if you don’t, add this to an autoexec.cfg. It’ll help reduce stuttering and decrease map load times when going from one map to another.
echo Executing autoexec.cfg...
//Stuff from Quantum's Little Config. (Thanks, Quantum!)
datacachesize 128 //GeneralDataCache in MB. Use 64MB for 1GB, 128MB for >= 2GB
mem_max_heapsize 2048 //MAY CAUSE INSTABILITY. Has to be set higher because of CommandLineCommand "-heapsize"| Maximum amount of memory to dedicate to engine hunk and datacache, dont user more then half of your Memory
cl_forcepreload 1 //Force texture preloading
echo Autoexec.cfg executed!
mem_max_heapsize may cause instability, so if you get instability, try commenting it or setting it lower (say, 1536) and seeing if you still get instability. If you continue to get instability, continue to lower me_max_heapsize. Do not set it lower than 256 (its default).
Also, add this to your launch options:
-noforcemaccel -noforcemparms -noforcemspd -noipx -nojoy
If you use a joystick, remove -nojoy.
I hope this helps.