I remember the reasoning for it (300px) but just forgot who told me. The error that you are pointing out is also documented [1] and has a patch [2] awaiting review.
In the bug that I had mentioned [3] the image loads just after a very long time. Are you sure that it takes only an extra second? What is the effective delay for the user? Could it be more?
I would imagine the answer is in PageImages, which I'm unfamiliar with. Have you tried contacting Max, who wrote it, about this issue?
I haven't yet, I'll talk to him soon, thanks!
--prtksxna
[1] https://bugzilla.wikimedia.org/63207 [2] https://gerrit.wikimedia.org/r/#/c/121607/ [3] https://bugzilla.wikimedia.org/63156