Ookla's speedtest gives the option of running their speed test using a single-threaded or multi-threaded connection.
According to this source:
Most large downloads [over the web] and streaming services operate over a single connection to the server, so it makes sense to measure the throughput available over a single connection. Personally this is my preferred test when comparing ISPs even though it may not show what the connection is capable of with multiple simultaneous connections.
Why would single-threaded connections be the preferred choice when comparing different ISP speeds? I'm guessing to test the quality of an ISP's connection, right?