As I understand the information provided on
Firstbeat’s performance condition is just a real-time version of their VO2max fitness level detection minus your base-line VO2max. You get some insights on their VO2max fitness level detection in
Especially, page 3, calculation step 4) should explain some of @mcalista’s observations.
That said, if your performance condition reading is negative throughout each workout, it should update your estimated baseline VO2max, unless you deactivated its auto-detection. At least this is my understanding.