As somebody that gives injections everyday in the medical realm, I give injections quickly. If giving an IM (intramuscular), you should have the right needle guage and depth. Let the needle do the work for you. No need to suffer and go slow unless you are into pain. Now...by that, I mean go quickly in. Once in, make sure you deliver your full dosage and then pull it out. Going in slow can be painfully agonizing AND YOU CAN ACTUALLY LEAVE A LESION (WHELP) in the area. Just do it all in one motion. I have seen lesions on people (these are red ecchymoses...bruising) when they go to slow. Just my two cents for what it is worth.
Geoff