Why does Reaper not time stretch loops as good as Acid Pro 6 (Examples provided)

interesting :(

It must be the acid loops, because the time stretching algorithm is Elastique in both cases (I believe version 2 for Acid Pro, and the choice of 2 or 3 for REAPER). Acid loops are pre-sliced to accommodate tempo and pitch changes.

I've done a few tests on your material from 120 to 100 and for me, the best algorithms are elastique pro 3.23 and the new Rubber Band library with percussive detection. elastique 2.28 doesn't do very well on this percussive material, but I'm surprised you say that they don't do it either, as for my ears there is a very audible difference. In fact, I think I get similar quality as your Acid example. Granted my test is less extreme as yours. Stretching from 75 to 120 is a bit of a stretch (pun intended) as it is a bit extreme. All algorithms will break down with extreme stretching/compression. But if you had given the original loop, i could have reproduced your test.

I tested E2/E3 and it's modes quite a long time ago and don't remember all the details. The conclusion I came to is that efficient/synced mode is the best for percussive material, of course ymmv...

Yes, but for REX loops. Not sure if it's supported for Acidized WAV.

You can try, but it's not a realistic expectation to see it done right in the next version... Acidized WAV is recognized as media with embedded tempo information, though, however there's no handling of slices like with REX files (there are even options for REX in Preferences).

But doesn't Reaper feature an automatic slice function for that purpose?

Sure, but it doesn't guarantee best results without a lot of tweaking. Besides, if slices already exist in the media, why not just use them already? Can be done for REX, so why not for other sliced audio formats?

Look for dynamic split in actions. Nonetheless, this wouldn't solve the issue, as these slices wouldn't behave as they already behave for REX. You could create stretch markers with that function, but again, this is stretching, not moving slices (as Acid WAV/REX do).

Because it CAN loop?