@nono240 - In general, using USB attached storage does not work well with ZFS, (and TrueNAS). Their have been a few, (as in more than 10), instances where a USB attached storage device caused problems for a TrueNAS user. (To be fair, those 10 times were over several years…)
There are other issues and problems with USB, thus I wrote about them here;
Sometimes using a USB for the boot device works okay. Especially if you save your configuration often.
I am not trying to stop you from using the NUC or USB attached storage. It is just that if you have trouble, the answer might be to use hardware that is more NAS than miniature PC. Or to use different NAS software that does not use ZFS.
As for the speed issue, I don’t have a direct answer.
But, do you have a known working USB 3.x speed device?
And does it work at USB 3.x speeds on the NUC?
The output you show clearly has 480Mbit/ps, Spd=480, so a USB 3.x speed device should show at least 5Gbit/ps.