Calibre's pretty snappy on my laptop, with 22934 titles in it. To be sure, my machine is a fairly beefy one with an i7 CPU and 8GB of RAM behind it, but I do believe that there's a definite speed increase since the v1.0.x releases became available.
Personally re: this new library manager--I'm willing to try a new thing, but I'll happily wait till there's a beta or actual release version to try. Meanwhile I'm sticking with Calibre, since I'm fairly well invested in its ecosystem by now. Definitely need to clean up my library though: it's rather haphazard in the extreme in spite of the valiant effort of Calibre's database to make sure things are right, books and metadata-wise. Tags in particular have gone from messy to mutated alien jungle full of carnivorous plants. Might just nuke it and start over properly before I get to 100k books and lose the will to clean things up permanently.
