There are all sorts of reasons for managers to dread performance reviews. For one, it’s a lot of work. In preparation you need to request people to send peer feedback, write peer feedback for others, and write your own self reflection.
Then comes the real fun part: the judgment, the writing, the calibration with other managers, and then the delivery. At this point, you think you’re done, but no — throughout the remainder of the year, you need to deal with the aftermath.