"Inconsistent performances" refers to the situation when someone or something doesn't perform or behave consistently or predictably over time. It means that their actions, abilities, or outcomes vary and are not steady or reliable. Full definition
However, in recent seasons the pair have been guilty of inconsistent performances, showing brilliance one week, and mediocrity the next. (arsenallatest.com)
Even with higher specs still get inconsistent performance between different Android devices. (bgr.com)
Its leader's shaky, inconsistent performance does not help. (politics.co.uk)