Surf Soho MK3 not up to speed?

I bought a Surf Soho Mk3 6.6.3 build 1068 Hardware Rev 1 about the time it came out.
I was using a cellular modem USB as WAN. It worked great.
Last year, we got 100Mbps cable and started using the cable modem wired as WAN. No complaints.
Recently, Spectrum upped the speed to 200Mbps. Last week, we had the cable tech come to the house due to cable outages. He replaced some connectors and our Walmart splitter and got all 4 upload channels working and proceeded to do speed test. His tester got ~240Mbps as well as the modem output. Wiring the modem to the desktop computer gets ~235Mbps. When we wire the modem to the router, then wire the router to the computer, the computer speed drops to ~90Mbps. The wireless speed near the modem is ~67Mbps.
I had always assumed that when an ethernet product was advertised 100/1000, that it would operate up to the 1000 speed. Same with the wireless speeds. I just saw where the throughput for this unit is 120Mbps. So, I have a couple of questions:
Why are my speeds not closer to 120Mbps? Should I expect them to be?
When shopping for consumer grade routers, I don’t see throughput mentioned. Should I expect anything close advertised speeds for ethernet? Wireless?