Two sources: #1 at mba[dot]com site, gmat online system requirements and #2 when you run the system check software test, it will run an internet speed check. I am not sure if test taker can proceed with the exam should the check fails.
#1
At the system requirements, there is this para in table that explicitly states - Internet Connection: "A consistent connection speed of 1 Mbps down/up is required. We recommend testing on a wired network as opposed to a wireless network. Tethering to a mobile hotspot is explicitly prohibited."
I can't post a link as I am a new member. The link below, "." is obfuscated with [dot].
www[dot]mba[dot]com/exams/gmat-online/prepare-for-your-exam/system-requirements
#2
goto home[dot]pearsonvue[dot]com/gmac/onvue ; scroll down to
"Run system
test Be sure to run your test on the same network and computer you will use on exam day."
Click on "test my system"