The performance review process has a small impact on salary.

The promo process is not based on value or difficulty, but on the size of the organization that one is running. This is also true for higher level ICs, except they do not manage people, but rather manage/lead projects (which then have a certain amount of people involved).

Here's a rough breakdown:

  - L4 -> 1 person
  - L5 -> 1-3 people
  - L6 -> ~7 people
  - L7 -> ~25 people
  - L8 -> ~70 people

The approximate 3x difference between the levels is also found in other organizations, for example in armies: division ~= 3 brigades, brigade ~= 3 battalions, battalion ~= 3 companies, company ~= 3 platoons.

Misunderstanding this is the source of almost all frustration with the promo process. This process is designed to build and expand the organization, not reward awesomeness. There are of course deviations from the simple schema I listed here, but this is the hard reference point.