Let's liken it to an engine. If it won't start, it isn't performing poorly. It is not working at all. However, if it starts and the fuel mixture is wrong, it will run but splutter and burp and not perform well.
I hope that clears up my position on the issue.
Back to brakes, If there is air in the system, they will fail or, if you quickly pump the lever, they will work and perform well. However, if there is oil on the disc, the lever will work but the brakes won't perform.
My definition of perform in this case is "mechanically (hydraulically) sound but tribologically poor".