Miserably slow, I had hoped that thanks to the higher read rate the write rate would also be higher than with the usual USB stick, but unfortunately this is not the case.

It's quite slow, just did on CrystalDiskMark 8.0.5:
SEQQ8 317/137
SEQQ1 369/168
RNDQ32 8.73/0.69
RNDQ1 6.72/0.61

... have loaded >500 GB, duration: a few hours, but the read rate is still such that videos can be played without any problems.

No idea. But bad. Takes ages. Every turtle with hip problems is faster...