Optimal drill speed involves maximizing how fast you can drill through a substance while getting a smooth hole and not damaging the drill bit by over heating. The correct speed will allow the drill bit to cut the material without getting too hot. Too slow will "chew" or tear the material. Too fast will cause excessive heat. Spade bits are especially problematic if run too slow.
The best rotational speed depends on the type of drill bit and the material being drilled through.
As a general rule:
- The larger the drill bit diameter the slower the speed.
- The harder the material being drilled the slower the speed.
Here is a chart for a twist drill bit:
Note: Hardwood ratings are for hardwood species with a Janka number below 1500. For hardwoods with a Janka number above 1500 use slower speeds.
There are other tables for other drill bit types at the link above.
You can find a variety of tables with similar information at other sites.
If you are drilling at a recommended speed and find that things aren't working well you may need to do some experimenting to arrive at the best speed for your situation.
Speed charts assume that you have a sharp drill bit.