Found this thread while researching the same issue. I know this is a year old but, wanted to comment incase anyone else came across it. After a bit more research found this in the release notes from version 100.1.0.9.57 under "Resolved Issues":
Added a warning message informing the user that the SpeedFlex tool built into
the P300 UI is unable to accurately measure the maximum throughput speed
due to CPU limitations, and therefore users should use SpeedFlex to verify
connectivity only, or to use an external platform to run speed testing. [FR-2706]
Hope it helps