Ardour 6 internally always runs at speed 1.0 (or -1.0, or stopped 0.0).
There is no vari-speed that scale "BPM" or "n_sample" time progression
per cycle.
Instead Ardour 6 vari-speed mechanism transparently re-samples I/O.
So process-time is scaled only relative to wall-clock time.
From a plugin's POV this is similar to "freewheeling": The plugin
processes data as if the host plays at speed 1.0. While the host
plays this data at a different rate.
Some plugins may like to be informed about the host's actual
playback rate.
Currently this is mainly for the benefit of github.com/x42/repitch.lv2.git
6481437f0a