My leviton countdown timer switch makes my bathroom exhaust fan rotate much more slowly than a regular switch. Why?

If this is a new installation, I believe you have a Leviton dimmer instead of a timer. For example, Leviton countdown timers like the 6260 and 6230 have times printed on face, while the similar-looking Leviton dimmer has no times.
Look at instruction sheet that came with product and make sure it says 'timer' and not incandescent 'dimmer'

If this installation worked before and suddenly stopped, then timer has gone bad.
Use circuit analyzer and check nearby outlet to make sure line is clean. Ground faults and circuit noise can adversely affect electronic timers.

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Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.