I've been observing what causes this, and have come to some conclusions.
Firstly, turning hardware acceleration off or on makes no difference.
The issue seems to be connected with the drawing of lines. When the slowness has kicked in, if I am zoomed in enough, I can see that is the drawing of a line that takes a long time. It becomes fatter, slow step by slow step, taking way too long, until the line has reached the desired weight. Each 'step' can take a second or more. This affects lines that have been drawn using the pen tool, and also lines which have been specified as the border or decoration of a paragraph. If I move a text block that has a paragraph with a line as a border, it can take a long time to select the text block, to move it, and then for it to draw. The same delay applies when nudging an element using the arrow keys.
But here's the fun part, and I think there could be a clue here - if I disable the UI, the slowness disappears. Everything works as fast and as snappy as it should. Then if I restore the UI, the slowness comes back instantly. The slowness is somehow being caused by an element in the UI. I've done this many times, and I have established it to be the case. As soon as the slowness starts kicking in, I go into 'no UI' mode, and work there.