Part of the reason I'm asking, XFinity just "upgraded" our speed like all customers*, and so far performance has been noticeably worse. When I speed test** I am indeed getting the new nominal speed, but sites take longer to load, and there is definitely more buffering and low-res when we stream now. We've even had 2-3 complete outages for up to 12 hours since the "upgrade," but our CSR tells us the outages are infrastructure, unrelated to the upgrade. It's only been a month or so, we'll wait and see, but so far not so good.
Yes, I've rebooted our modem and router several times.
*If something sounds too good to be true, it probably is.
**I'm am a little leery of online speed tests, I assume the ISP recognizes speed tests and performs accordingly just for the test?