Debugging this : In the project where I first saw the problem, I had a MIDI track with >100 unflattened take lanes - set this to Archive and playback started instantly. Then I began culling the take lanes: I am pretty sure the delay in starting playback is proportional to the number of MIDI take lanes, as the delay got shorter as I removed more. The instrument happened to be Cakewalk SI Electric Piano. But I think it will occur with other instruments.
On another project, the delay was maybe only 1sec, and the same thing happened. I also think flattening the comp helped.
But, what I think could be an actual bug, is that when freezing a midi instrument, the number of takes lanes is still affecting the p/b start time. I would have thought that while frozen, the midi events on that track are irrelevant as we are just playing back a bounced audio track?
And I am fairly sure this got worse with 2025.06