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booklover81 
theducks - see what happens when you 'hide' the library as he suggests
It's probably irrelevant but following on from Saberdeyn's post...
I'm currently building a library of scanned comics. At about 6000 books, Calibre's startup times went from 10-20 secs to a couple of minutes. I enter metadata into the library list (spreadsheet) cells, and about the same time I noticed that tabbing from one metadata cell to the next had developed a noticeable lag.
So after investigating and experimenting I 'homed in' on a one of my custom columns as being the likely cause; it was a union (concatenation without duplicate names) of four names columns. So I removed it - the startup times went back to < 10 secs with no significant lag when tabbing between metadata cells in data entry 'mode'.
I 'knew' from what I've read here that Calibre's performance can suffer if you have columns created from other columns, so I'm not surprised that my column did just that. But its interesting that it did not gradually deteriorate to any significant degree, but when it reached its tipping point it practically 'fell off the cliff'.
I can live without the column during library 'construction', might put it back when its finished.
BR