Just to summarise the resolution of this problem:
Originally Posted by phil_ga
I will look into this, but i remain unconvinced at this point that it actually changes the opf content rather than the file date.
No I do not have that feature set. ...BUT there is a switch in the same menu called "Limit the search metadata". I tried that. It makes a huge difference to response time when searching direct from the search bar with free text! Wow. Thanks for the tip!! Now I will look into the sluggish behaviour in other areas - to see if any of them are also sped up (later - bedtime now...). But this tip is a big help with responsiveness for vanilla searching. Thanks!!!
My slow and sluggish calibre is showing consistently faster performance in all areas I previously complained about. itimpi 's tip did the trick!
In the preferences under: Preferences -> Interface -> Searching
Choose: Searching -> Limit the searched metadata
I set my values to: title, authors, series
Not only is searching from the search bar much much zippier, but the sluggishness in other areas, such as editing fields directly also disappears.
Gee, I would think this setting should be "on" out of the box - but then again, not a lot have databases this size (but i am betting there will be more and more).
Thanks calibre, thanks itimpi and jackastor!.