The core is this advanced PC with music files on internal drives but editing was wickedly slow until I cleaned out files in the SSD C drive AND, possibly more importantly, unchecked the property in the drive “Allow files on this drive to have contents indexed in addition to file properties”. It could be that any indexing of files gave another unnecessary hurdle for Roon to cope with. Acce ss is now virtually instantaneous 
More computer savvy Roon users please add your opinion.
Edited title for clarity - hope this helps someone else.
This may well help. If Microsoft is really trying by default to index the content of music files, this is a new level of stupidity. It’s not hard to exclude file types for which it makes no sense! I never would have thought that one should check this.
(Unless it’s indexing just tags, but then that can be done on lower priority so that it doesn’t interfere with other tasks)
I have found that taxing the database when on a Windows core has a hangover effect that only re-starting the core software will remedy. Searches, using tags, those types of activities create some resource issue that doesn’t resolve without the restart. Also seems now to be affected by what may be happening on Roon’s own servers.
The trouble is far less on my Linux Roon core. It also seems to recover after running slowly without needing to be restarted, which is what causes me to suspect that some of the slowdowns are on the Roon server end at this point.