Having your tape deck service periodically is a good idea. Not only can
they clean the heads, but they can adjust the head azimuth. If you
listen to tapes you mostly record at home, bring one in to the service
department when you bring the deck in. They should be able to optomize
the azimuth angle to give the best high frequency performance with your
tapes. Its kind of like a custom tuning. One other thing, DON'T try to
demagnatize your tapes, you'll erase them! Its the head that would need
demagnatizing. None of this will solve the speed problem, that's mechanical.