No my IBD is finished and my node is stopped. So essentially my RPi is connected to an SSD which has the whole bitcoin-core structure, but bitcoind is not running.
Which SSD do you have exactly?
I'm just guessing here: is it possible you're using a very low quality USB cable?
Have you tried
Gnome Disks? It has a Benchmark feature.
Or have a look at
GSmartControl, see if it can give you a reason for the low speed.
I also ran fstrim -v on the mounted directory and it trimmed some data, but it didn't work afterwards. I mean no amelioration was seen.
That shouldn't matter for "
hdparm -tT", it doesn't use the file system.