Speed less than half of advertised on MB8600

Hi!
I recently moved into a new rental and installed a brand new MB8600 (DOCSIS 3.1) with the 800 Mbps plan.

Even directly connected to the modem over Ethernet I basically never see anything more than 400 Mbps. Speed tests typically come in around 100-200 Mbps, sometimes less.

The house used to have a 5-way splitter leading to each room, and sometimes the modem wouldn't even connect at all. I removed that and installed a new amplifier splitter instead. The modem now connects reliably, but the speeds are terrible. I also use MoCA devices in the house to connect different rooms, but I have consistent speed variability and issues even when no MoCA devices are connected.

It is an older house and I am wondering if the coax is bad. Attached is the connection information from the modem, but I don't know how to interpret it. Can someone help take a look to see if this might suggest bad wiring in the house, or some other issue?

Thanks!