There's a built in way to check how fast qc runs on an engine, in case you're optimising things. The command is "profile", and it lists the top 10 functions in terms of commands executed since the last profile command or server start. As an example on unpatched quake:
e1m6 at 72fps
425616 PlayerPreThink
270814 WaterMove
241542 PlayerPostThink
202689 ImpulseCommands
177480 CheckRules
165648 CheckPowerups
125843 ai_stand
94928 StartFrame
76144 door_touch

Things worth noticing:

*ImpulseCommands is certainly up there. Almost certainly the most important optimisation would be to skip everything if self.impulse is 0. I'd agree with doing that check before calling the function ImpulseCommands(calling functions costs a few operations).
Further optimisations would be the else if Lardarse suggests, and storing self.impulse in a local float at the start of the ImpulseCommands function - to avoid looking it up each time, which is more expensive if you have to go through a layer of self.something first. But optimising the very rare cases where there is a change of weapon isn't gonna yield anything like the benefit of skipping the whole thing in 99.9% of frames.

* CheckRules places highly, even though that function is entirely pointless in single player games. I think the calls to cvar are pretty expensive here.

* Results from this kind of assessment will be different at different framerates. The playerprethink type functions occur every frame, but since monsters think in 0.1 second slices, that's only 1 in 7 frames. Playing at 15fps, I'd imagine ai_stand could top the charts.