I finally had time to test the settings in the database. Yes it makes a total difference. After I set "db.fuzzySearchEnabled" to "0" the issue disappeared entirely. I typed for two or three minutes without stopping and when I stopped the screen was completely up to date.
A question @laurent - since this has fixed the problem will I suffer any issues if I leave the setting at "0" for the time being? I do not use search much, if at all, in Joplin, at least not regularly, but I do add notes several times a day. If that setting is solely concerned with "fuzzy search" then I can afford to lose that functionality until you have identified the underlying problem. If this will have wider effects then obviously I will set it back to 1.