David some msps like being pumped fast... If it clears the slot by enough you do not gain anything by pumping slow.
Part of the reason pumping faster seems to get a little more velocity has to do with the seal. Especially the crosmans, because they have the cup style seal. I think you get a tiny bit of leakage on every pump, and by pumping faster the cup style seal seals the tube better, sooner. Plus i think you might be right about the heat as well, Rob. Other than that, whether you pump fast or slow, I think as long as you have a consistent technique you will have consistently good results.