Slow Hardware (Archer C7 as switch / AP)

Hardware Questions and Recommendations
1

I have a UniFi setup with Gateway, 16 port switch and an access point.
I wanted to use my TP-Link AC1750 Archer C7 as an additional switch / access point.

When I plug my laptop directly into my 16 port switch I get 200Mbps from fast.com.
When I plug my laptop into a dumb D-Link switch which is connected to my main switch I also get 200Mbps.
However, when I plug my laptop into the TP-Link C7 which is connected in the same way as the dumb switch I only get 100Mbps.

I have disabled DNS, DHCP, Firewall on the C7 and still it is slow. This happens on stock firmware as well as OpenWRT.

Is there something I missed?
I removed the WAN interfaces. I set a static IP for the LAN interface and set the gateway to be the IP of my UniFi Gateway.
Everything seems to work, it's just very slow.

2

Go to Network -> Switch and check the negotiated Port speed for the connection to the PC and the connection to the Gateway.