After installing TB3 on my 4gb Sandisk Cruzer micro I found it to be really slow in terms navigating between e-mails or deleting messages/feeds.
I found a few posts about this and went through defragging and deleting MSF files which both helped for a few hours. I also have indexing switched off.
I now believe I have fixed it by changing the USB mode.
Within device manager find your USB drive
Right click on it and choose properties
Choose the Policies tab
Ensure "Optimize for performance" is chosen
I am in the habit of always ejecting the usb drive before unplugging it so I suggest making this change only if you are also in that habit.
So far so good, It is now much faster to navigate. Fingers crossed it stays this way.
Let me know if anyone else finds similar results.