Its the cable. After changing the cable between the Switch and the Router, i got iperf test up to: 0.0000-10.0416 sec 1.09 GBytes 932 Mbits/sec.

We had some workmen in the house a few weeks ago. Apparently, they damaged the cable. The test cable was unfortunately too short for permanent use, but it was enough for testing. I now get an upload of 65MB/sec on an average. This is about the same as the limit I had already determined through hdparm on the RPi4. I don’t know, why hdparm is faster directly on the laptop, maybe the USB Controller is not the best on the RPi4. But anything is better than 11MB/sec.

The situation is different when the laptop is only connected to the network via WiFi. Here I get a maximum of 28MB/sec. I have not yet found a solution how this could be faster.

It seems the limiting factor is the switch or cable. If i connect the Rpi to the router directly, i get: 0.0000-10.1222 sec 281 MBytes 233 Mbits/sec

The speed looks fine for me on the Laptop itself:

/dev/sda:
 Timing cached reads:   28080 MB in  1.98 seconds = 14161.28 MB/sec
 Timing buffered disk reads: 448 MB in  3.02 seconds = 148.55 MB/sec

 /dev/sda:
 Timing O_DIRECT cached reads:   604 MB in  2.00 seconds = 301.42 MB/sec
 Timing O_DIRECT disk reads: 454 MB in  3.02 seconds = 150.36 MB/sec

On the Pi itself i get:

/dev/sda:
 Timing cached reads:   1974 MB in  2.00 seconds = 988.54 MB/sec
 Timing buffered disk reads: 196 MB in  3.02 seconds =  64.91 MB/sec

/dev/sda:
 Timing O_DIRECT cached reads:   122 MB in  2.01 seconds =  60.64 MB/sec
 Timing O_DIRECT disk reads: 194 MB in  3.03 seconds =  64.03 MB/sec

This also much above the speed, i got, when i transfer files via SFTP/SCP.

I have tested also iperf and got: 0.0000-10.2100 sec 113 MBytes 93.1 Mbits/sec

It seems like this is really somewhere limiting to 100MBit.