The pi has a slower cpu, slower network, slower memory, and slower disk devices than a conventional computer.  The network device on the pi3 can't even keep up with a regular desktop's network, and is slower than 1G.  It is designed to be cheap and robust, not fast.  There is nothing a pi cluster can do faster.  In some circles, a pi cluster is considered a joke -- although a pretty cool one.  Several pi clusters have been built for demonstration purposes.

The pi has more readily available I/O pins than a regular computer, so at a stretch, a pi cluster could do a lot more gpio.  This might actually be useful of the pi's were distributed over a wide area; they could act like cheap (and slow) distributed I/O extenders, but this is not a conventional cluster. by any stretch of the imagination.

Los Alamos National Laboratory's Pi cluster's purpose is not to be fast.  It is to test cluster algorithms.   Sometimes you *want* a slower computer when testing code, because if it is too fast, things happen too quickly to debug.  But also, 750 pi's is a lot smaller and cheaper and uses a lot less power than 750 conventional servers.