YAPT (yet another performance test) is a benchmark recommended by a pair of drive manufacturers and was incredibly difficult to locate as it hasn't been updated or used in quite some time. That doesn't make it irrelevant by any means though, as the benchmark is quite useful. It creates a test file of about 100 MB in size and runs both random and sequential read and write tests with it while changing the data I/O size in the process. The results are a good look at overall drive performance.
This is not a 4k aligned test, and the issues the 840 PRO used to have misaligned writes appears to have vanished with the 850 PRO. That must be some extremely responsice and low latency flash!