TechDudeDon't set the launch option.
The in game cvar mat_queue_mode tells the engine whether to enable threading.
Yes I know, but the -threads option is/was supposedly used to force the Source engine to use # of threads (in conjunction with mat_queue_mode).
Example would be someone with quad-core CPU would use -threads 4 to help the Source engine spread out the load over the 4 cores instead of just hammering 1-2.