Well, as a beginning whistler, I figure all these tunes were meant to be danced to. Consequently, I like to practice new tunes while walking. I start out at "geriatico" aka "andante with a walker" on the assumption that by the time I've gotten the tune down to a brisk and steady walk without stumbling, I'm pretty good with it. It amazes me how my feet and fingers can correlate a tempo without me having to think. I can't do that with a metronome to save my life.

Just my $0.02, fwiw...

V
ps as for changing the tempo the midi files, it depends on what software you've got and how proficient you are with it...