When I started working on complete overhaul of KleanSweep GUI (moving from KDE 3 to Qt4) I soon faced problem with slow QTreeView performance when using my own item model. To make the long story short, with around 1.6 mln records the QTreeView was completly unusable taking tens of seconds to redraw when scrolling its contents. I googled a lot only to discover other people had similar performance problems with Qt Model/View architecture.
I almost gave up, until my recent discovery: the uniformRowHeights property of QTreeView. It indicates (if true) that all items in the view have the same height - and allows the view to perform some optimizations. This solved the performance problem I was facing! So now my SQLite-based model works fine and the new KleanSweep (still W.I.P) will be capable of displaying millions of files with little memory footprint!