This is going to be good... What is your home network setup?

Here is mine... I have Xifinity also and from the wall I have a Arris modem and then a Linksys router that provides the home Wifi and 4 Lan ports.

Also, what are your speeds like using speedtest.net or if you google speedtest and use their tool?

Also, what are your speeds like using speedtestdotnet or if you google speedtest and use their tool?

Not sure if your first reply is recommending that I add equipment, but that's not an option, at least not right away. I'm only subletting one BR in a 2 BR apartment, and the Xfinity account and the routing HW are my roommate's/landlord's and he's been out of state since early December due to Covid and I don't know when he'll be back. As to the speed tests, I've been using 3 different ones: bandwidthplace.com, speedtest.net by ookla, and all-nettools.com. I used the results from all-nettools in my original post, because they've been the lowest and also show the most variation. Speedtest.net has been consistently in the 80's with little variation, and bandwidthplace has been in the 60-80 Mbps range. All-nettools has ranged from 2 to 25, and last night was hovering around 3 to 5.

Here are my latest speed test results, obtained ~ 12:45 pm PST on Mon. 12/28:

                                2 Browser pages open           1 Browser page open
                               (test page, email page)           (test page only, after closing email)

speedtest.net 86.62 (& 5.99 upload) 86.22 (5.99 upload)

bandwithplace.com 40.15 (5.60 upload) 84.03 (5.62 upload)

all-nettools.com 12.31 16.74, then 8.94

As noted previously, speedtest shows almost no variation, always in the 80's. Bandwithplace shows considerably more variation, but it also frequently reaches the 80's. And all-nettools never reaches the 80's (I don't think it has exceeded 25), and it often goes down into the single digits. Don't which of these is most relevant for my issue, but there's the data.

Going back to your setup question, it's very simple: WiFi router supplied by Comcast. No Arris, no Linksys. If I open Device Manager and open Network Adapters, five items are listed: Intel 82577LM Gigabit Network Connection Intel Centrino wiMAX 6250 Intel Centrino Advanced-N 6250 AGN (2) Microsoft Virtual WiFi Miniport Adapters

.....

I see my reply to your equipment question posted, but not my reply about the speed tests. So I'll answer your latest questions, and then double back to post latest speed results. I tested a LAN cable last week, and the results seemed similar with and without it.

                                Cable               WiFi

speedtest.net 87 87 bandwidthplace 36 to 53 48 to 84 in various tests all-nettools 11 to 27 5 to 21

To me, speedtest.net would seem to be the least useful because it shows so little variation & stays high. But I know very little about these tests and what they're actually testing.

New piece of data: Just tried to open my online broker's web page in Firefox. The url appeared in the address box, but the rest of the page was blank. The page never loaded (and it never even gave me a timeout message). Just a great white space. But then I attempted the same operation using Chrome, and the broker's sign in page appeared immediately. It's only one test, but it suggests there's differential behavior between Firefox and Chrome, at least for this one website. I tried Chrome because last night Firefox exhibited the same behavior (blank page) with this same website, and the broker's Help Desk mentioned that their site is optimized for Chrome, not for Firefox. Bottom line, it may be a Browser issue, not a WiFi or ISP issue. What do you think?

Can you send a similar screenshot. see screenshot Something may be blocked so make sure to check 3rd party cookies.

Don't think screenshot will help, because the problem is intermittent. Just loaded broker sign-in page immediately using Firefox. Earlier in the day page wouldn't load as I said; and before that, page loaded, but quote requests timed out frequently. Also, I cleared my entire Browser history within the last two days.

Did you try with a network cable?

Here's at least part of the problem identified. I got in touch with my online broker's technical support desk. The man on the phone walked me through Firefox options. When we got to Privacy and Security, I told him I had checked the box for "Always send websites a signal not to track me". He suggested I change that setting to Send a Do Not Track signal "Only when FF is set to block known trackers". As soon as I made that change, the broker's sign-in page that would not load and had left me with a blank white browser page loaded immediately. So it looks like a have a choice: Be able to go to my broker's website OR Always send Do Not Track signal - but I can't do both. I will continue to monitor overall speed issues, and see if they also resolve with this one change or not. But clearly the sign-in page will not load with the "Always" Do Not Track option.