rsync extremely slow, huge amount of TCP Dup ACK

In our network I have several PCs which sync files from an rsync server (without encryption) to their local hard disk. Often (not always) it is really slow. In sum it took 33 min for 2 GB to transfer on a GBit network. The whole package of ~8 GB / 300000 files took more than an hour (I didn't measure the whole thing).

The server is on 10 GBit network. The clients and everything between is on an 1 GBit network. The server's and the client's CPU and RAM usage is low.

I setted up a virtual machine doing the same and sniffed the traffic using Wireshark. It came up there is a huge amount of TCP Dup ACK packets. Nothing happens in the transfer but the TCP stack is sending continuous Dup ACK packets with the same sequence numbers. The sequence number graph shows it.

Ping times are good (< 1 ms). What can cause this behavior? How to fix it?

EDIT 1:

I also tested it with a client from the same subnet. TCP Dup ACK occurs too but only a few times. The transfer is much faster. I got 1,5 GB in only 1:45 min. So I think it must be a network related problem. Maybe a tcp setting in the OS or a router problem(?)