Sync 3 screen slow and laggy but fine with USB stick in?

[Ripp]

Had a bit of a nightmare getting my Sync 3 software and maps updated but I'm pretty sure they're both done now, the software took a bit of faffing around and the maps were a nightmare, I had the USB stick plugged into the van for 2 weeks and it would constantly keep telling me that part one was installed and that I needed to key cycle to install part two, but eventually after a couple weeks of trying to complete a key cycle I got in the van this morning and by some miracle I got the 'Installation Complete' message pop up! But now I have an issue...

The whole time I was trying to get the maps to update the whole Sync screen would be slow and laggy (like having to wait 10 seconds after pressing a button for it to register) but while the USB was inserted and the 'Updating system software' message was on the screen the Sync screen was fine, fairly fast and button pushes worked ok. So I figured this was something to do with the fact that the software was up to date but the maps were not (as I couldn't get the update to complete) but even after having the 'Installation Complete' message and Ford's website showing that my software and maps are up to date and doing a master reset the Sync is so laggy it's unuseable.

But the very weird part is, if I leave the USB stick inserted with the last map update still on it the Sync is fine, no lag??!! So now every time I get in the van the Sync turns on and tries to install maps even though they are already up to date, so I get the whole 'Updating system software' message, then after a couple of minutes get the message saying 'Installation Complete' and have to press 'Ok' to continue on with my day with a non laggy screen.

What on earth is going on here, because as soon as I remove the USB stick it's lag city again?