In Ardour 5 I am able to simply slide the playback speed slider ("shuttle speed control") to start playing at >1x speed, and after releasing the mouse button it keeps on playing at that speed.

In v6+ the sliding part still works, but upon releasing it the slider springs immediately back to 1x, so I can't easily select a playback speed and keep it going. As a workaround there's the varispeed option, but for me it's not as intuitive (it uses semitones instead of "Xs" as its unit), and feels much less flexible. I could probably live with it if I was forced into it, but I much prefer v5's behavior.