What Can I Do to Fix My Parents’ Awful Wifi Speed?

I'm really struggling with my parents' wifi situation, and I hope you all can help. Their internet speed is incredibly slow—like pages taking over two minutes to load! I don't really understand how all this works, so I'm reaching out for any advice.

Here's their current setup: they have a coaxial cable connected to a 2-way splitter, one line goes to their cable box and the other feeds into a Spectrum modem (E31U2V1). From the modem, there's a Google Nest H2D router. Then, there's an Eero N010001 connected to the router, along with two Eero extenders in the same room as their desktop.

Previously, with a different Spectrum modem and just the Google router, everything was pretty quick—pages loaded in a couple of seconds. But after upgrading the modem, the speed tanked. They switched to the Eero after Spectrum recommended it, but setting it up has been a headache; it was supposed to replace the Google router but couldn't connect wirelessly to the existing network. So I ended up connecting it via Ethernet to the Google router, but the speed hasn't improved at all.

What steps can I take to resolve this? I'm open to any suggestions—thanks!