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.