unfortunately the timestretch algorhythm in the OT is way subpar, compared to software that’s been on the market for years.
it’s a shame, but the very dated yamaha SU700 does a better job at timestretching 
the least-hassle way would be prep your stuff in ableton or reason or some other daw.
OT’s internal routines work with small changes (And by small I mean +/- 2bpm, no more) and it’s still kinda meh.
for me, reworking the timestretch would be one of 3 crucial steps to make the OT a dream machine.