The same way one does it with 'clean' code: Using profiling tools. Security is a bit less straight forward, but still.

>will feel very objective.

Keyword: Feel. And while most people could probably agree on terrible code being terrible, the 'less terrible' the code is, the more this argument becomes a feeling. And then we hit up point where it's no longer possible to discuss things using objective arguments - how will a senior java developer, who is used to heavy OOP style coding reason with a senior C developer for whom such OOP heavy style is the oposite of 'clean'?

Ofcourse, the example is (too) simplistic, but even in this thread You have people arguing about "big functions" vs splitting things up. And unlike the performance, which You can alaways just point to the raw numbers, here You can't rely on any sorts of 'objectivity'