Success! After tweaking the RJ45 plug at the switch end I'm now consistently getting over 800Mbps on my cat5e wired connection: ~50' of wire from the router to a gigabit switch + another ~25' of wire from the switch to a wall jack + ~6' of patch cord from wall jack to computer.
Yay, glad to hear you found the source of the trouble!
I didn't suspect Ethernet cable issues because you reported more than 100Mbps speeds. Generally, a cable will either do Gigabit, or 100Mbps (100-Base-T). So when I hear folks reporting 90Mbps-100Mbps speed tests consistently, I think they've got a bad Ethernet cable.
Another category of slow-down is Ethernet dongles that connect via USB. I've seen those performing at 200-400Mbps, and everyone is confused because it's hard-wired Ethernet, right? But the USB (2.0 maybe) won't go that fast.
But I'd say that PC/browser and particularly antivirus software slow-downs are the most common sources of degraded performance for wired systems. These can often be isolated by testing with another system, disabling the antivirus, etc.
And of course, WiFi won't go full Gigabit, yet - and getting folks up to speed on the limitations of WiFi is another challenge for us. WiFi performance will range from 300Mbps to 600Mbps generally, depending upon the client device and access point. How fast the WiFi goes will vary by client, old ones might only do WiFi-4, or 802.11N, and see far slower speeds. Some might not support 5Ghz, or not be configured to use it. And clients with 1x1 antennas will be slower than 2x2 antennas. But finding all these specifications can be a challenge.
But the good news: Unless we've got an equipment issue, the fiber just does gigabit. The source of slowdown can almost always be isolated in the local area network. And that's great, because now your ISP isn't the issue, it's a local one that can be resolved. Maybe it requires that a 100Mbps switch be replaced with a gigabit one, or that a client device be upgraded or replaced. But your internet connection is gigabit, and you can make changes if necessary on your side to access that full capacity.