Why are network speeds reported by Activity Monitor different than the ones reported by speedtest.net?
It's using a totally different averaging method.
It's not examining the exact start &amp; 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* &amp; 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.
After comments
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 &gt; Update Frequency