I'll tell her to go slow during head if i don't wanna cum quickly, say if she finds a sweet spot, but i can usually last well enough when getting head so it's rare for me to say to go slow.
With sex i'll just switch positions, do a hard kegel and see how i feel. If i am about to cum i'll let her go on top as i don't find that very stimulating, i can just like there and chill, occasionally groan so she thinks it's making me feel good. She's up on top doing all the work giving herself an orgasm, then when she's done i can roll her over and bang her hard as i've had a big rest.