When the performance of something significantly degrades over time.
The names comes from the Nvidia Kepler GPU architecture (GeForce GTX 600&700 series), which was infamous for suffering severe and steadily bigger performance losses in games after its release.
This was caused by
poor optimisation in Nvidia’
s drivers after the release of the
Maxwell architecture (GeForce GTX 900 series), as without shader optimisation the
raw performance of Kepler cards fell off a cliff.