We use CareerLark[1] in our office. It's nice and lightweight (plus easy to onboard people on to).
This quarter s/he's slacked a bit, maybe s/he's not as driven or it's something personal - but s/he's still performing just as well as the other two engineers who've both performed exceptionally well and I'm giving them both a 5.
Do I give the engineer I expected to do better a 5? Or a 4? I mean I expected them to do better but I'm still really happy with their work.