Lags when zooming
 

Aside from my studio, I'm running a second PT9 at home on my MacMini (Intel Core 2 Duo 2,4GHz, 8GB RAM).
Here I've got the strange issue, that during playback (only during playback) the zoom is lagging. There's a very profound delay from hitting "t" or "r" to performing the command, which makes the working experience kind of unpleasant.
I run the audio from an external FW device, set the playback engine to "1 processor 95%", and changing buffer size doesn't affect this behavior.
There are no plugins, VIs or anything else, and it happens in every session. One track with audio is enough to trigger that issue.

I ran out of ideas, could any of you guys help?