Ableton Live 12's Time Utilities let you reshape the rhythm of a MIDI clip fast. With nothing selected, changes apply to every note in the clip, so you can double or halve the speed of an entire pattern with a single click, instant half-time or double-time variations without re-recording anything.
For finer control, the stretch knob dials in speed changes beyond simple doubling or halving, letting you nudge a clip's timing to whatever feels right. Select individual notes first if you want the same tools to reshape just part of a phrase instead of the whole clip.