General Discussion

...who is doing the job well?

For example:

Members with seniority are allowed to choose their own offices and committee assignments. The latter is one of the most important privileges a member of Congress can earn because committees are where most of the important legislative work actually happens, not on the floor of the House and Senate.

Members with a longer term of service on a committee are also assumed to be senior, and therefore they have more power within the committee. Seniority is also usually, but not always, considered when each party awards committee chairmanships, the most powerful position on a committee.

The seniority system enhances the power of the committee chairs (limited to six years since 1995) because they are no longer beholden to the interests of party leaders. Because of the nature of the terms of office, seniority is more important in the Senate (where the terms are for six years), than in the House of Representatives (where the terms are for only two years).

https://www.thoughtco.com/what-is-the-seniority-system-3368073