"Start with resetting window layout from the View tab." This alone seems to have restored the speed I'd normally expect. Thanks!!! but what's the background to this. Just for my info?

I'm just a Studio user, not a programmer, so TBH I do not really know, what is wrong, BEFORE you use this function.

But Studio has a very complicated window layout, even if this does not seem to be so at the first glance. However, when you take a closer look, even the default layout just out of the box does contain a lot of overlapping windows, some even on auto hide. And when you customize it - in my case this is heavy customization I would say, dividing Studio on two screens (would I have 3 monitors, I would certainly divide the program on all of those) - things are getting more complicated. Reset window layout brings the settings to what is defined in your user profile and makes Studio work again properly, if nothing within the settings has been damaged. I keep a daily backup of my Studio setting files - templates and profiles. This way I can return to yesterday, if something is wrong. And I recreate the profile for the new Studio version usually from scratch.

So in general this option simply restores some internal connections I would say - which is usually enough to have Studio running smoothly again.

Your file is a MemoQ export and should work very fast in Studio. Just for information: a mqxliff file with roughly 300,000 words and more than 90% of repetitions opens in Studio slower than in MemoQ. But once opened, an autopropagation of a segment with ~150 repetitions uses 90 seconds in MemoQ and "only" slightly less than  60 seconds in Studio. There are over 28,000 segments in repetition, so if I calculate statistically 28,000 / 100 segments I have at least 280 repetition cases, 30 seconds less each in Studio. Makes 140 minutes or at least 2,5 hours less waiting time for program reaction. In the end of the day, even if it does not seem to be so, Studio is pretty fast.