That doesn't sound like time-stretching at all, that just sounds like slicing and arranging the slices to playback at a given BPM. I guess it's a crude form of timestretching, and btw it can be done just in Renoise's regular sample editor, you don't need to find it in the "Phrase Tool" or whatever you're referring to...
Usually, at least with modern software, when we say Timestretching we are referring to an algorithm-based approach, like those found in Ableton Live and other DAWs, or like the Time Machine Pro inside Kontakt, etc. These alter time without affecting pitch, and vice versa. Currently, Renoise does not have a timestretching algorithm, and this kind of thing simply can't be done in that fashion without using other programs. The "slice to fit bpm" approach however has been around forever, and as you've said, it gives mixed results depending on tempo / how close a match, etc. You can get way more than 64 slices, though. Slice a sample, use the S commands (usually best to create the first and last commands, then interpolate between them to automatically get every value in between).
But to be fair, if you're using Ableton and you're sampling rhythms or rythmically, you'll want to use "Beats" mode anyway. Because like El HYM said, anything else is going to mess up your transients (and you might need to manually finetune the transient markers, cause they often seem to end up right on top of the transient, but maybe that's me being anal).
Anyway, you can reproduce this behaviour in Renoise using slices: Place slice markers where you'd have your transient markers (or use the auto-slice button, but I never use it so I have no idea if it's any good). Then, without worrying about pitch for now, sync it to your BPM. Optionally prepend silence to be in time with whole beats and append more silence until it's a nice power of two in length. Now you right-click and do "Render Slices to Phrase". Doublecheck if the phrase indeed plays the sample in time to your beat (if not, check the Render Slices Options). If it does, you can reset the pitch back to normal (or however you want to pitchshift). Also, the various quantization options give you control over warping.
That's the basic setup for Ableton's "Beats" mode timestretching. The default setting for "Beats" mode is that all slices have a full ping-pong loop set, and a short attack and release so that they somewhat fade into each other. If you're familiar with the other "Beats" timestretching settings, it should be self-explanatory how to adjust the Renoise setup. Not entirely sure if you can exactly reproduce the percentage slider, but with the proper modulation envelopes you can get very close. But then, this is Renoise, so you can now do so much more For instance edit the phrase, add note-offs to cut slices prematurely, pitch shift, retrigger, FX, whatever.
And yeah for the other kinds of stretching, the rubberband tool works excellent
Edited by triple zero, 14 January 2018 - 01:28.