While you're undergoing cancer treatment, you're going to be too fatigued to worry much about your cognitive performance. I wouldn't expect to be able to distinguish lower productivity as a result of "chemo brain" from lower productivity as a result of all the non-cognitive side effects until several weeks after your active treatment is done.

Taking off as much time as you're able to rest on and after treatment days will go a long way to helping you feel better the rest of the time.