I panic whenever I read stuff like this. What is an under performer? How do I know if I'm one? How can it be expected that everyone in an environment is a spectacular code ninja that does utterly awesome stuff all the time?
it's not. you just have to find a range of tasks that you are able to perform at a high level at, and then make sure that your job is doing it, with confidence, day in and day out.
if the link in the other comment is true, it seems like their senior salary range is 200-300k/year. in that range, the excuses stop, and the results begin, or you're fired, have a nice life.
in the real world it actually begins at 100k but technology skews this figure much higher since basically anyone with a pulse and a keyboard can make 100k in this industry.
So shit like this is why so called performance management is bullshit. Performance is not doing what is in the best interest of the team or company. It is whatever is in the best interest for your manager.