Recently DL speeds seem to drop with server distance

I have had X1 Basic, with rated DL speeds of 85 Mbps, for 2 years. I bought a new Netgear CM500 (16 channels) in January, set up the ethernet (it's a wired connection) and the XF tech activated it. At the time, I was getting DL speeds of 85, sometimes more, from servers around the world. I use the Internet to tutor students in Moscow, so the speed matters.

Beginning in mid-March (quarantinaville), those speeds dropped dramatically, down to 12 Mbps to Moscow. I let it go, thinking it was the streaming traffic jam. But those problems have persisted to today. I ran a few Speedtests to Comcast servers, and got the following:
Seattle (home), 89
Spokane, 89
Boise, 88
Billings, 76
Cedar Rapids, 43
St Paul, 45
Chicago, 40
Detroit, 33
Richmond, VA 28
And, of course, Moscow, Russia: 15

I checked the modem report for errors, unusual frequencies, mislabeled channels, etc. All is in order.
Of course, I have rebooted the modem many times and many ways (power out, XF reboot, Netgear config page reboot). It makes no difference, nor does the time of day.
Pings and upload speeds are all constant, and there is no loss in 200 ping iterations.

My hours with XFinity tech support has been a waste of time. Yesterday, the tech did something she called "provisioning", which helped for two speedtests, then it immediately returned to these numbers.

Is there anything I can do, short of shopping for yet another modem?
Is this the new truth about Xfinity? Is it time to consider their rival for fiber optic?