Max Bandwith between two PPPOE-Clients

Hi,
I have created a pppoe-server on a ccr1072 and connected the pppoe-interface via 10G fiber to a switch.

A speed test with iperf3 between two pppoe-clients, which are attached via 10G copper to the switch, never exceeded 3,4 Gbit/s.

Is that normal behaviour?

All settings of pppoe-server are default.
To exclude a perfomance issue of the test machines i did an iperf3 test directly between them on the same switch which resulted in a speed of 9 Gbit/s