Thank you, Isaiah.
I shall certainly try what you kindly suggest.
In the past I have been badly bitten by moving Addons in and out of their appointed folder; I’ve assumed that RW keeps its own internal check/manifest on what it expects to find there; and that manually ‘going behind its back’ is asking for trouble.
I learnt when I upgraded to RW 8 today (which is when the slowness started) that there is a new Addons installer. And wasn’t/am not sure how that works and whether it’s now a requirement to follow (that or) a specific procedure to install (new) Addons.
In the past, as you’ll remember, methods have varied… double clicking the distribution file(s) for Addons, dragging it/them to the RW icon in the dock, running a dedicated installer and simply moving it/them into a specific folder.
If it’s now safe to move Addons around, I shall gladly give that a try 
.
I assume you’d suggest moving everything out except those four (viz FormSnap, Houdini, Translate and the Text Stack)?
You’re right: I have a very fast machine - an iMac Retina 5K, Late 2014 with 4 GHz Core i7 processor and 32 GB RAM + plenty of HD space!
It was also b/c I’d read of others experiencing the same or a similar phenomenon that I posted, assuming that it was a shortcoming of version 8.
Thanks for your encouraging suggestions, Isaiah!