@Ernball
Not a BSD solution but something you might check to isolate your problem ..
Some time ago I had an "argument" with my ISP concerning my BW throughput..
and had to "prove it wasn't my equipment"...
One of the steps was to
hookup the Windose box directly to my cable modem to
"show" the tech that the problem
"still existed" and
it was NOT my OpenBSD firewall..
I would suggest that you try that..
I used speed tests to check mine and tried to do the tests about the same time every day to
get a "baseline"..
Oh and the ISP gave me 6 months credit on my business account when they finally found out it was "their" problem.. not mine.. My neighbors thanked me for my efforts
The ISP did not listen to them since they are non-technical "residential customers." ....
hth
rk