I had a section of code which incurred ~20 clock cycles to make a function call to a virtual function in a critical loop. That's over and above potential delays resulting from cache misses and the need to place multiple parameters on the stack.

I was going to eliminate polymorphism altogether for this object but later figured out how to refactor so that this particular call could be called once a millisecond. Then if more work was needed, it would dispatch a task to a dedicated CPU.

This was an incredibly performant improvement which made a significant difference to my P&L.