If it's not a synchronous disk access, i don't know what it could be, really. So try and delete/rename these .db files and see if it changes anything.

It can't be network, because it was made asynchronous in the relevant parts (asynchronous DNS resolution and optional network thread).

Now that i think about it, it could be your fonts. Like suggested in the readme, edit conf/font.conf and remove the mossys:fonts/_ttf/ line. These fonts cause heavy substitutions in fontconfig when a glyph isn't found, and it could take several seconds in some cases, totally blocking the UI in the meanwhile.