It seems a fair discussion point, even if you don’t agree that the payoff is worth the effort.

I don’t read it as a negative thing, and for me <500kB seems reasonable. Page size is an aspect of design – even if many websites are suffering far greater needless bloat than this one.

Performance is a feature, or at least it should be, of any good design.

Of course it might be a very low priority feature, so it would be valid to close any issue reports as "WONTFIX" if the devs have no time or feel they are better spending their time on other matters, especially if people on slow connections are not expected to be a significant part of the target audience, but commenting on potential performance issues is perfectly valid when discussing a site (re)design IMO.