Drill Fast vs Drill Slow
Most of the time you'll be better off using slow speed, high feed: i.e. slow drill speed and press hard. Bigger the hole the slower the speed. Small holes you might get away with a medium speed, anything over 10mm you need to be real slow. Judge your speed by your swarf. Nice long curls are perfect, but you'll need good bits. 

Oh, and don't forget to use lube. Water in a squirty bottle gets used a lot around here if the proper stuff has gone walkies. 

Drilling dry overheats and kills bits faster than anything.

Remember to dry off though if you don't want rust.