The HL engine is not thread safe. You can spawn a thread to do a job that doesn't modify the game at all (like SQLx does), or if you're very very careful you could spawn a thread to read data (like creating bot waypoints or whatever). But you can't safely modify anything in the game from within another thread.
Stuff you do from within a thread will still use the same amount of CPU, but it won't block the game server from continuing until it is done.