This question is considered on-topic on Programmers.SE, and I don't understand why.

It is a question about computer science, not a question about programming or programmers. It's something a good programmer should know, but it's not programming (in the same way, using a computer is definitely required of a programmer, yet computer usage isn't programming). It's a subject where an expert would be a computer scientist, not a programmer.

Furthermore, it's an objective question. The run time of a function is a perfectly well-defined notion (at least in a given context, and the choice of context isn't relevant here). There may be more than one way to compute it, it would still be the same thing being computed — this isn't about making a design choice.

So, going by the site FAQ as well as the overall tone and audience of the site, this question doesn't belong here. I don't think there's currently a site in the Stack Exchange network that's really appropriate for it¹, but the closest, and officially-sanctioned, is Stack Overflow.

Why, then, is this question not migrated to Stack Overflow?

¹ _{Reminder: CSTheory is about research-level computer science only.}