Recorded some MIDI without setting a proper project tempo, didn't you?
Tutorial
Upload a MIDI file from your computer
On the top-right corner of each track, click on the Scale BPM button to scale the track
Apply any transformation to any track you want
Export the final result
Note: The app works currently best on SMF0 MIDI files. Future updates may add full support for SMF1 and SMF2 files. If you're interested in this, please let me know in the GitHub issue section.