Yes maybe it depends on this! Did you use deltatime to slow down other things in the game?
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As far as I am concerned, the songs speed shouldn't be affected even if I didn't use delta time.
I snooped around my code and it's quite probable, that I didn't account for the timers I use for the appear animation for each tile. That would explain the feeling as if the song was slowed down since all tiles would be somewhat off beat. Of course neglecting the fact that they were off beat in the first place since I placed them manually making the margin even wider :P
Once again, thanks for pointing it out. In future games I will definitely fix this issue!