As ever, thanks for the comprehensive response!
I did a little experimentation but couldn't avoid the slow-downs other than by restricting the amount of memory Windows uses at boot: and it's a workable solution as long as I remember to change it back! Assuming it doesn't automatically reset. I haven't noticed a precise pattern to the slow-downs but I am still getting then in Act II.
It seems that the good news is that it's more difficult to make a mess of my build than it was in DD! It's actually interesting to see how many new ideas and changes were implemented in BD that we still see in D:OS. One of the main reasons I was wondering is because I'd contemplated retraining to use a sword, which turned out to be a huge liability for my mage in DD, though as it's turned out, the bows are so much more effective in BD that I can probably continue to be a ranger-type character quite happily.
I'll work my way through the guides as I play, as I did in DD: thanks for digging out the links for me.
Oh, and while reading a bit about the Divinity timeline (and is it correct that Aleroth grew from the healers' village we saw in DD to the city in ED in just 82 years? That said, IRL 82 years is plenty for the same thing to happen IRL and for a city to feel like it's always been there so I don't know why I'm surprised) I think I may have inadvertently managed a massive spoiler for myself:
is it true that my handy death-knight is Damian?