There are plenty of performance questions on this site already, but it occurs to me that almost all are very problem-specific and fairly narrow. And almost all repeat the advice to avoid premature optimization.

Let's assume:

• the code already is working correctly
• the algorithms chosen are already optimal for the circumstances of the problem
• the code has been measured, and the offending routines have been isolated
• all attempts to optimize will also be measured to ensure they do not make matters worse

What I am looking for here is strategies and tricks to squeeze out up to the last few percent in a critical algorithm when there is nothing else left to do but whatever it takes.

Ideally, try to make answers language agnostic, and indicate any down-sides to the suggested strategies where applicable.

I'll add a reply with my own initial suggestions, and look forward to whatever else the SO community can think of.