That's a misreading. "Max. Link Rate" on https://www.linksys.com/us/p/P-WRT54GL/ refers to the max wireless speed no matter what brand or model of router, never wired. I have only been speed testing wired with Cat5e & Cat6.
Do a text search for "54 Mbps" on the manual: http://downloads.linksys.com/downloads/userguide/WRT54GL_V11_UG_C-Web.pdf
And you'll see chapter 1 says: "The Router lets you access the Internet via a wireless connection, broadcast at up to 54 Mbps"
The wired ports are 10 Mbps/100 Mbps & the speed tests were run with every other device on the LAN off, so full speed should have been allotted to the wired speed test computer.
Through my previous WRT54GL, back before the upgrade notice above, with the same wired computer, I would sometimes get within 5 Mbps of the 50 Mbps previously advertised.
I can try swapping out the Belkin Cat6 cable running from the modem to the router, for the Cat5e running from the router to my computer.
If that doesn't work, I need to give my family more options than buying another router almost immediately after this one. Since the "upgrade" to 100 Mbps was free, can I get a discount on my internet service if the max download speed is reverted to 50 Mbps?
