"Hmmmm, tricky..."
It appears (to me) not to be 19.3.2369 per se. It may be a dual problem with 19.3.2369 but with a Microsoft update influencing matters?
As I was still having too many problems, even with Avast uninstalled (autochk/chkdsk worked but the machine was still running badly), I put my backup on and was surprised to find I had 19.3.2369 before the backup. I thought I had (a) backed up, (b) updated MS updates (c) updated Avast in that order, but now it appears the other way round? My memory of the order in which I did things is at fault.
Checking logs, I updated Avast first, late Friday night, then set a backup going overnight. Then I did a bunch of MS updates on Saturday, see .png attached - note I excluded the two Mar 2019 updates.
The problems started after that and I initially suspected Avast due to the Autochk issue, as it seemed those MS updates wouldn't be messing with the disc at a low level. I removed Avast and Autochk/chkdsk worked. But then the machine was still very slow so I put the backup on.
So I think 19.3.2369 is part of the problem, but it may need one of the MS updates to reveal it. I'm now on my post-19.3.2369 and pre-MS-updates version and the machine is fine (touches wood)
Thanks
W7 Pro 64 bit