AirPort speed

When the “tower” style of Airport came out I bought one for it’s 802.11ac, which was supposed to be faster than the previous protocol (“n” I think). I have stuck with it, but since TimeMachine over wi-fi has become progressively more unreliable it has lost utility. Today I had Bell 4G cellular internet installed (fibre and cable not possible) which is advertised as providing 50Mbs. So one of the first things I did was to plug the Airport into a LAN port, as I have my printers set up to connect to it. The second was to run Speedtest while connected to the Airport, and then when connected to the Bell modem.
Airport DL17/UL10
Bell DL53/UL10
The Bell modem/router also uses 802.11ac, so I was surprised that it is so much faster. Could its LAN port be crippled by a 10base-T connection? Needless to say I have retired the Airport and connected my printers to the new SSID. File transfers over the LAN are much faster too: a 1.5GB file took 9 minutes to copy from my M2 MBA to a 15" 2018 MBP this morning, and when I tried it again this evening it took 1.5 minutes.
I’m not complaining at all! Just a bit puzzled.

A while ago, due to flaky wifi performance, I disabled wifi on my Time Capsule (Tower Airport with internal hard disk) but kept it connected to my LAN with ethernet.

It still serves as a 2nd Time Machine backup for a couple of Macs and I also have an old external hard drive connected to its USB port for extra backup of video files, photos etc. I don’t rely on the Time Capsule for primary backup.

Having the Time Capsule on the LAN doesn’t seem to affect overall ethernet performance.

I have two Time Capsules that are functioning just fine, one at a relatives and the other at work.