I found time to poke around more based on your great suggestions I've tried to standardize on using the "manual" 100 mb test. Also, I did what you suggested and tested speeds of various Domain name servers and concluded that my ISP (Comcast) was consistently good but that a Google servers equally good. So designated it as the secondary domain name server. That change didn't seem to help with the lethargy of rendering webpages).
Here's what I've observed and what I've concluded:
1. I've tested three browsers (Firefox, MS Explorer, and Edge) ack-to-back. . They seem to produce comparable speed test results. Also tried pining the same web page on each browser. Similar lethargy. So I don't think I can blame Firefox.
2. I've tested my desktop using both a Wi-Fi dongle to connect to the router as well as a hard wire connection. Speeds via Wi-Fi seem consistently slower (40-60 mpb/s) than the hard wire speeds 90-100+ mb/s) . No surprise there.
3. I've done speed test using my wife's laptop which uses a Wi-Fi connection to the router. Her Wi-Fi- speeds are comparable to the speed obtains (i.e. 60-70 mn/d). Yet her laptop will render identical pages must faster than will my machine.
The whole reason for going thru all this is my very slow website rendering. Abysmally slow on many occasions. It seems to have trouble quickly rendering graphics or embedded videos. (slow to "pop"). Thus I began this saga of testing of download speeds on MyTest.com. Interestingly (and importantly) my wife's laptop, with test speeds about the same as my machine when on wi-fi, renders identical pages like crazy whereas my machine (whether on wi-fi or hard wired) can't even come close in the speed which her laptop renders them.
This leaves me with the conclusion that it's something with my machine. Yes, it's a bit older (Gateway Pentium dual core 2.6 GHz, Windows 10) but...... rendering speed is clearly getting worse and is dreadful upon occasions. I've run all the usual things, Spybot, Malware Bytes, etc. Nothing identified. Even made sure Firefox was a clean install (no add-ons). Same lethargy.
I hate the thought of upgrading to a new machine. Yes, I can do it. But . . .. . not unless I just have to. Are there any other tuning suggestions or should I just bite the bullet and get a new machine?
Thanks for your sage advice.
P.S. Love your speed testing site. One suggestion. It would be neat to be able to be able to log a "Note" on each speed test to give some info, e.g. that you'd tweaked something before the test, which machine, how connected, etc. This would really help remember which test did what and maybe why.