I'm sorry for not contributing with hard evidence but I've seen this behavior (though I never heard about it from anyone else before now)
These days I rarely need large bandwidth to any single host, but have a lot of bandwidth use between many hosts including streaming applications. Bufferbloat is far more apt to be problematic under this workload than ISP priority, unless there is a bandwidth shortage on the local links.
Cloudflare's - https://speed.cloudflare.com
Cloudflare's speedtest often performs worse for me.
iperf3 - https://iperf.fr Is the most accurate CLI speed test I've found (depending on which server you choose).
The best speed test is to download some large linux ISOs with 100+ peers, so you max out your connection to tons of networks.
Downloading packages with $PACKAGE_MANAGER, downloading files from $CLOUD_STORAGE, downloading videos from youtube, downloading linux ISOs, receiving files through $INSTANT_MESSENGER, ... they all are very close to the speed I get with fast.com.
I like this one because your speed really depends on what you're downloading and this one tests some of those scenarios.
They can legally do that as we don't have Net Neutrality anymore.