This is from launchpad: https://bugs.launchpad.net/nautilus/+bug/350150

'I uses sometimes obexfs to get files from my phone to computer via IRDa. This has one "feature" - any file transfer is very slow, roughly few kB/s. When I open some directory on such fs in Nautilus, it tries to download every file in folder causing double transfer of every file - once for thumbnailing it and second time for actual download. Side effect is also freezing it's UI.

Nautilus should prevent something to do this. I could imagine two approaches:
1) blacklist potentially slow fs's. Apart from obexfs, remote fs's can have unpredictable speed and also removable fs's connected throught slow interfaces, like USB 1.1
2) check if getting file for thumbnailing doesn't take too long time. If it does, stop making thumbs of files in that particular directory

It would also be nice if thumbnailing run asynchronously, so that any problem here wouldn't cause UI freeze.'