With thumbnails enabled, thunar becomes unbearable slow on ptp (via ghoto2:// and local thumbs) and mtp (via mtp:// and thumbs on all files) devices. I suspect, that each time the view changes, eg. by scrolling, a new retrieval of (possibly) all thumbs is triggered. Until this is completed the ui stays frozen. On local folders or network folders via sftp:// this works differently in two ways: (1) Thumbs are fetched only for files currently visible in thunar, (2) For thumbs already retrieved the cache is used. Given the lower io speeds on ptp/mtp devices this approach should be adapted for ptp/mtp devices as well (if possible).
This is my biggest (only?) issue with Thunar because I frequently need to copy pictures or videos off my Android phone but Thunar becomes painfully slow to use as soon as I have a MTP tab open that is displaying a folder containing images and/or videos with thumbnails. Disabling the thumbnails isn't an option as I don't know what is what otherwise with all image and video filenames just being a date stamp. I hope this will be fixed for the next XFCE release as I think this is a common use case for Android users.