Cool about the M12! Yes the internal memory is limited, but as far as the Ableton question.....
Are you running a Left and Right out, into a mixer, then a USB from the mixer into your laptop as a live setup?
Some other thoughts...….
Esoteric loops that don't have a lot of timing factors involved.....you could load samples directly into the App, but as far as more in-depth/longer samples, it might be causing some of the latency/lag problems.
Also, Make sure the Multi12 is communicating with the iPad correctly.
If your still having issues, I would say to just load samples directly into Ableton, and not the M12 App. This is probably the easiest solution...…..
Hopefully that makes sense, let me know how its going Robin!