Thank you all for the very helpful suggestions - really appreciated. I've just cleared the cache and it's certainly improved noticeably! (Thank you @PeterMacKenzie2 and @DanPatterson ). I'm also exporting a layout to start a new project (this is taking a long time... hopefully it will be done soon!).
I'm using locally stored data (on an SSD drive) so it's not that, and am already on version 2.8, but good suggestion @AubriKinghorn .
I will run the ArcGIS Pro Performance Assessment Tool as soon as I'm clear of my current deadline (thank you @TomBole - good suggestion!) I'll also work through the other suggestions in the link that @DanPatterson provided.
Thank you all, really appreciate the help!