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, measuring the speed of the file transfer along the way. Each test takes 15 seconds, so when you have a fast Internet connection, a huge amount of data can be transferred in that time. Be careful when using the speed tester on a cell-only connection, as using it frequently can lead to high data-transfer bills.