By default thunar/tumbler tried to thumbnail network shares. This causes multiple large issues: Its slow. For example lets take a share with a lot of videos. tumbler often gets stuck trying to thumbnail them requiring it to be killed for it to continue thumbnailing. It makes the share impossible to unmount, because tumbler is stuck trying to thumbnail many videos on a network share. this behaviour is very far from optimal, thunar/tumbler should do what nautilus does, not thumbnail network shares. Or at the very least don't try to thumbnail huge files on a network share.
With Thunar 1.6.x, you can select to generate thumbnails "Never"/"Local files only"/"Always", so I think this is what you want.
I'd rather be able to keep thumbnails for local files though. having no thumbnails for my pictures folder would be really annoying. What I suggest is either: Do what nautilus does, and by default thumbnail local files, but don't thumbnail files on network shares (for example samba shares) or At least do what PCmanfm does and give us an option to be able to limit the max file-size of a file to generate thumbnails for globally, this way I could have it thumbnail images but not videos which would be fine with me. The current behavior just isn't good at all. I can only either have no thumbnails anywhere, or have my network share become unusable because tumblerd chokes trying to thumbnail large videos over the network...
Oh, I glossed over your mention of local files only. So you say this option is introduced with the newest thunar? In that case great news :) That would be perfectly acceptable for me.
Yes, it has been introduced in Thunar 1.5.3, here's the commit: http://git.xfce.org/xfce/thunar/commit/?id=3ef31277ffe1abe842bc5d2374883a92425a8de9. You can change the option in > Edit > Preferences… > "Display" tab, "Show thumbnails" combobox.