When I run a test on speedtest.net, I get 90Mbps for download and 35Mbps for upload.
Looking at Activity Monitor I see 50 MB/s and 17.5MB/s.
My understanding is that 1Mbps equals 0.125MB/s.
Why is there a discrepancy?
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It's using a totally different averaging method.
It's not examining the exact start & stop point that Speedtest is, it's just doing an average over the past 5 seconds.
I got 217mbps Speedtest, 20MB/s Activity Monitor. It took it at least 2 updates to even reach that, by which time Speedtest had just about finished.
Switching Activity Monitor from its default 5 second update to just 1 second* & the values just about match - I was getting 217mbps Speedtest with 27MB/s Activity Monitor (roughly).
Also bear in mind you are comparing a one-trick pony - an app dedicated to providing line speed results - against one that's more a general overview of what your entire machine is doing.
As there was still a discrepancy at 1s sampling, try to eliminate sources of error - switch from WiFi to wired Ethernet; remove any VPN connections; test another account or Mac.
*View menu > Update Frequency