I’ve completed a reformat and I’m pleased to report things are, thankfully, MUCH better 
This looks like it was an issue with a gem - so not Apple’s fault…
This has been fixed too 
Fixed as well 
So it looks like this, in conjunction with their fixes in the latest release, sorted it (doing the upgrade alone did not sort out the issues - it needed to be a clean install - and why I still maintain that we should do this every major release (maybe not till the third point release tho!))
It still doesn’t feel as it did when it was brand new mind, and so there may well (probably?) be some element of deliberate throttling going on. They still need to do a little bit better imo - there’s no reason why this machine, for simple tasks like opening apps shouldn’t be as fast as it was on day one.
This is still the case for the Affinty series of apps - but it appears this could be partly the fault of Affinity as I have seen others complain about the same thing and on multiple platforms (appears it started after a specific release).
Wouldn’t surprise me however that the Affinity devs are themselves a little stumped due to some weird ‘update’ Apple has made.
Overall though I am MUCH happier, but still feel they need to keep improving the performance side of Big Sur - things have definitely improved significantly (which isn’t difficult considering how bad things were!) but they still have some way to go imo.