Brief lock-up when switching to PLAY MIDI track

I just started using PLAY 4 (was on 3) and Cubase locks up momentarily every time I switch to a MIDI track linked to PLAY (mouse moves but nothing on screen responds to clicks). It never did that with PLAY 3 and it doesn’t do it with any other VI. I can disconnect the MIDI tracks from PLAY and the problem goes away, so it’s definitely related to PLAY.

You can see it in the VST performance meter - the bars bounce around as they normally do but when I switch to a track connected to PLAY, they momentarily stop moving, then start bouncing around again when the cursor comes back to life.

Anybody have any ideas? I’m using the latest PLAY download and the latest Cubase 7.5 download.