Very slow painting on some machines

Ive had a very small number of reports of very slow screen refresh from some Windows users.

In the past, I have known this to be caused by cascade redrawing: (controls too close to each other causing unwanted paint events.)
But I thought I had caught all of those, and I dont see this slowness on any of my test machines, including a VM throttled to 1 processor and less than 1Gb RAM

Sadly the 3 users reporting this are not a great at answering questions, so Im struggling to find a common cause.

I do have a theory that it may be due to the recent change that puts images into a separate resource folder.
(I admit to bias: I really dislike this recent change in the way Xojo builds Windows apps)
Might this be causing fussy virus checkers to slow things down?
Are we hitting a bottleneck by loading the images on demand?

What else could be the slowdown?

Video cards with slow GPUs or not much memory, but that would probably affect all of their apps.