Better routers are equipped with a speed adjustment that allows for speed adjustment typically ranging from around 8,000 to 24,000 rpm (revolutions per minute). The reason is that not all router bits can be safely or optimally run at the same speed. Larger router bits have more mass and therefore a potential to create forceful vibration at higher speeds.
It's important understand what router speed actually measures. "Revolutions per minute" is simply a measure of the number of times that the bit makes a complete revolution in a given period of time, and not a measure of the speed at which the body and cutting surfaces of the bit are actually traveling. The outer perimeter of a 3-1/2" diameter bit is actually traveling significantly faster at 24,000 rpm (and much faster than it should be) than a 1/2" straight bit's cutting surface would be at the same shaft speed.
Saw Bits
Manufacturers often supply maximum free-running speeds for the router bits they sell. The maximum speed for a given bit is the maximum speed at which it can be safely operated, and may not be the best speed for the task. There are other factors to consider: router horsepower (a less powerful router will slow down when it's driving a large bit through a cut), feed rate, quality and condition of the bit, and the cutting properties of the material. It is impractical to give a list of specific speeds that will work well in every situation.
In general, a faster speed is more desirable than a slower one, for the simple reason that a faster speed provides more cuts per inch, and more cuts per inch normally yield a smoother cut. Using sharp, high quality bits and using the correct feed rate go hand in hand with bit speed in making a clean cut. Make practice cuts with a new bit until you are able to produce a smooth, even feed rate that is neither so fast that it produces a rough cut, or so slow that it results in burnishing and burn marks.
The chart below offers speed guidelines for the maximum safe free running speed of bits of various diameters. This is supplied for general reference - manufacturer's guidelines should be consulted:
Router Bit Diameter Maximum Speed
Up to 1" 22,000 - 24,000 rpm
1" to 2" 18,000 - 22,000 rpm
2" to 2-1/2" 12,000 - 16,000 rpm
2-1/2" to 3-1/2" 8,000 - 12,000 rpm