A traditional CPU is measured in actions per second. So 4GHz is 4 billion calculations per second.

It works be performing these actions on each cycle of an internal clock. And these actions are based in binary (0 or 1).

A qubit has more than two states. So by encoding a qubit’s states you can ‘use’ more bits per clock. It’s a multiplicative performance increase. And additional tricks/encoding can make it roughly exponential.