Slow rectangular selection in continuous view

Rectangular selection becomes very slow on large scores when in continuous view. It works fine in page view.

1) Open any large score (for example: https://musescore.com/user/496531/scores/4998592)
2) Switch to continuous view
3) Pan to the right so that the starting point is not in view (so that instrument names, clef and time signature are rendered in blue color)
4) Try to make a rectangular selection (shift + LMB drag). It'll be very slow and unresponsive.

PS: Any editing becomes slower, rectangular selection is just the easiest way to test this IMO.
It only happens in continuous view when the starting point is not in view.

Tested on v3.0.5.