Slow ethernet, despite GbE hardware: Searching for causes

I am having issues with slow network speeds in my home network between 2 boxes. Here are the specs:

Box1: ASRock H97 Performance, Manjaro

Box2: ASRock FM2A78M-ITX+, Debian 9

Switch: Netgear GS305v2

Both boxes are connected to the switch with Cat.7 (10m and 2m) cables. So everything should work with Gigabit Ethernet, right?

Yet running iperf gives this result:

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   105 MBytes  88.5 Mbits/sec   32             sender
[  5]   0.00-10.00  sec   103 MBytes  86.6 Mbits/sec                  receiver

Where do I start looking for the problem?

Edit: On one side I ran iperf3 -s on the other iperf3 -c xxx.xxx.xxx.xxx. I also testet file transfers via NFS and rsync over ssh, resulting in the same speed.

The output of ethtool on the debian machine:

Settings for eth0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Speed: 1000Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: on
        MDI-X: Unknown
        Current message level: 0x000060e4 (24804)
                               link ifup rx_err tx_err hw wol
        Link detected: yes

And on the Manjaro machine:

Settings for enp0s25:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  Not reported
        Advertised pause frame use: No
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: 100Mb/s
        Duplex: Half
        Port: Twisted Pair
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: off
        MDI-X: off (auto)
Cannot get wake-on-lan settings: Operation not permitted
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes

Looks like the Manjaro interface is setup for 100Mb/s only.

Enabling auto-negotiation for the network interface with ethtool -s enp0s25 autoneg on fixed it.

Thank you for pointing me in the right direction!