There is a word which means "decrease in performance", usually used in negative context. Here are some examples:
"The city's power station usually gives a steady voltage of 220V, but at 9PM, when the TV series ends and everyone turns on a teapot, the voltage may ... even to 215V".
"This algorithm solves a random instance of a problem in several seconds, but there are some specific instances on which its running time ... up to a minute".
"During the rush-hour the capacity of major streets ... significantly".
For Russian speakers: the exact Russian analog is "проседать", but I can't find a desired translation anywhere.
UPD: I've got one exact word on a tip of my tongue. Here are some words which may fit the gaps but are not what I need: sink, drop, sag, plummet, decrease.
Probably "degrade" is the best match I could think of.
degrade
is a verb and usually related to quality of something is getting worse (more like something damaged), but the quality does not necessary mean algorithm slowdown or decreasing capacity of something. More appropriate words to use arereduction
ordrop
(e.g. frame-per-second drop in video player) as mentioned in the answers below.