I have
this too with some bigger sessions. Mine are from
ardour1 and play just fine in ardour1. When opening them in
ardour2.[012] they play fine but continuously give me the "hd
not able to keep up" message. I tried removing unused tracks,
no change.
One version of that problem, involving deactive tracks, was fixed
by 2.3. I'm at 2.3.1 and not having trouble.