The app used a huge amount of mobile data when using the speed tester. Why?

There is nothing magical about the speed tester - the only way to measure the speed is to download and upload as much data as possible from/to the Internet and it measures the speed of the file transfer on the way. Each test takes 15s so when you have a fast Internet connection, a huge amount of data can be transferred during this time period. Be careful when using the speed tester with cell-only connection as using it frequently can lead to high bills for data transfer.