If I add a transpose track and draw an event on it, dragging the Project window horizontal bar handle or the selection rectangle in the project overview window becomes slower than usual. Changing values in transpose events is also very slow. When such a project is played, the cursor may move with abrupt jerks and the program may became completely unresponsive. On deleting transpose track, graphic responsiveness is restored.
Can anyone confirm that?
I had similar symptoms that cleared up when I went to:
Preferences → Event Display
Then move the Grid Overlay Intensity slider all the way to the left.
Try that and see if it helps.
I think this is a new graphics-related bug introduced in 6.0.3. I made a separate thread for it.
Bravo, orchetect 
! That did the trick. How did you figure that out?
My sessions were slow upon opening only after upgrading 6.0.2->6.0.3. I read the PDF file of new features and tinkered around with the settings until I found the one that fixed it. Like I said when I updated my post above, I think this is a new graphics-related bug introduced in 6.0.3. I made a separate thread for it. Hopefully SB will address it in 6.0.4…
Right. I hope so very much too. Meanwhile I changed the thread title to reflect the fact that a workaround is found. And again, many thanks for it.