And there is the magic term, performance consistency.
Some PC hardware review sites are doing performance consistency testing now, Anandtech being one of those.
Basically consistency is a measure of the times needed to perform multiple I/O requests. Think of the HDTune screens, with all the dots all over the screen, with (hopefully) most of them concentrated in a group, forming a line or bar horizontally across the screen. Each dot is the access time measurement of an I/O operation. Ideally, all the dots are concentrated together into a group of fast access times. Dots that are spread all over the display indicate very different access times for the same type of I/O.
The Anandtech consistency tests show the "scatter plots" of access times of a SSD, with and without over provisioning. With OP, the access time dots form a solid horizontal line or bar with a minimum amount of dots far away from the group. The dots further away from the group of dots are slower and thus inconsistent access times. Without OP the dots are spread all over the screen, with much fewer bunched together in a group of faster access times. The Samsung 840 Pro responds well to OP, and without it the access times are all over the display, and slower.
Of course, that is under extreme conditions IIRC, so not the usual PC environment. Still, looking at those graphs can be an eye-opener: