Certain websites and apps loading content SUPER SLOW

For the past 3 months or so, we've been having an issue at home where some websites and web apps will load content like they're on a dial-up modem from the 90's. Sometimes the content just won't load at all and the page will remain blank until I wait for a while and refresh it later. Sometimes it'll load up fast as usual, other times it'll fail to load again. It feels totally random. Even xfinity.com is affected! Perhaps the most frustrating part is, while one website may be loading SUPER slowly in one tab, I'll load up another tab and run an internet speed test, and it'll report our full connection speed like there's no problem at all!

It makes no difference if it's a phone or PC, ethernet or wi-fi, it seems to be affecting every internet connected device on our network. I've also tried changing DNS servers a few times, but it still makes no noticeable improvement. Just a few days ago we even replaced our xfinity modem/router with a newer model, and yet the issue still persists! At this point I think we have tried everything short of inspecting the wiring outside the house for damage. We're making an appointment to have a tech come out, but I have a hunch that they won't find anything wrong with the wiring, which hasn't been touched in years... and then we'll be back at square one with no solution in sight.

I've seen a couple of other posts in the forums from the past 5 years describing the exact same issue, but neither one had any solution posted, so I wanted to raise it again just in case there is some new information that could be passed my way. Using the internet has become a real pain lately, and I'd love to hear if someone else has the same problem.

[Edited: Removed 1st few IP addresses as personal information]