This is an ongoing flight details of my in-laws travelling from Frankfurt to India. I observe the speed of the craft is drastically reduced to 100mph and then picked up normal. Could someone please explain this?
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 :)