slow USB vtapes

I see low write performance to (modern) USB disks attached via USB 3.0

This leads to loooong backup windows and even DLEs somehow not written 
because things get stuck (?) somehow.

Until yesterday the disks were attached to USB 2.0 ports, I expected to 
solve this by adding a PCIe card with USB 3.0 ports.

Maybe the mount options aren't optimal:

UUID=3de7b456-019e-40ed-a1b9-aad41d371f26 /mnt/externaldisk7 ext4 
relatime,noauto,user 0 2

?

When I test things by doing something like:

sync; dd if=/dev/zero of=tempfile bs=1M count=1024; sync

I get values of maybe 130 MB/s or so, which is OK imo.

The old internal RAID delivers ~150MB/s (reading from the holding disk).

But with amanda (3.6.0pre2, btw) the flushes and dumps are slow.

Does changing some blocksize help with vtapes?

Sure, old hardware, I know.