What is this magical console command I speak of?

cl_forcepreload 1

And how does it work?

Game data is loaded in by the client as it's needed, not all at the beginning of a match. On slower less-powerful systems this can be a major source of sudden frame drops during gameplay. This issue is particularly noticeable while firing weapons and during explosions as your system struggles to load necessary information fast enough, resulting in a dip in performance.

The above console command essentially forces CS:GO to load the sounds, textures, models, and other game data it may need all at once before you load into a server. While this could theoretically cause your initial load times to be longer, it should reduce if not completely eliminate any major frame drops during gameplay (at least as far as information streaming is your bottleneck).

Note: This trick may not necessarily double your FPS, but should net some additional frames even on beefier rigs. However, as the comments have shown, your mileage may vary. If for some reason you experience a decrease in framerate or quality, simply type cl_forcepreload 0 into the console to revert the setting back to normal.

That's it, now simply enjoy the additional frames.