What made the below flight path to have very low speed during the middle of its course
What you are seeing is merely a glitch in the publicly available telemetry sites like FlightAware use.

They aggregate flight data from the FAA (ASDI), EUROCONTROL and others to feed their sites, but this data is not always accurate, and doesn't need to be.

For that interval, the speed was either malformed or reported as 0/unavailable (don't remember exactly) due to bad data.

The aircraft did not slow to 100mph or lower unless it landed :)