Hi Neil,

>>>>moving round the drawing and zooming in and out of the drawing, changing commands,
>>>>moving between DWG files all resulted in the 'Wheel'.
There is another bug in AC 2020, and I believe you see it on your side. To bring back AutoCAD speed you just need to remove all recent blocks from Recent blocks tab of Blocks palette. The "slowness" is the result of this bug with Blocks palette.
You can also set system variable BLOCKMRULIST=0 after you cleanup Recent blocks tab, this will prevent adding recently inserted blocks to it.