what kind of info are you looking for? In the simplest form - you need 3 inputs open to read the encoder. within emc you virtually hook up a few pins that connects the encoder to the guts of emc. If you are using the printer port for reading the encoder - you are limited by how fast the computer can count. Say you have a 100 line encoder on the spinde (400 quadrature) on decent computer hardware you should be able to count into the 20-40khz range. so - you should be able to count up to the 3000 - 6000rpm. As long as the axis can keep up with the spindle reversal - you are golden.
There is a lot of cheap interface hardware out there for emc that will move the encoder counting and pwm outside of the computer which doesn't tax the computer.