If you're experiencing lag, you should follow the instructions in this thread. The end result will give you a boost of around 10-30 fps, but I don't guarantee miracles if you have a Potato PC.
Put these commands into console in-game in the order that is here:
gmod_mcore_Test 1 (ONLY DO IT IF YOU HAVE MORE THAN 1 CPU CORE)
cl_show_splashes 0;r_cheapwaterstart 1;r_cheapwaterend 1;r_waterforceexpensive 0;r_WaterDrawReflection 0;mat_wateroverlaysize 4;r_ForceWaterLeaf 0;