It's not so much that certain programmers don't think that page load speed matters; it's that the culture around the platform doesn't encourage speed.

You're conditioned to not care anymore, because ASP.NET Web Forms makes it so damn hard to achieve responsive web apps, and building responsive web apps almost universally means breaking away from the standard ASP.NET Web Forms style of development. You have to abandon pretty much everything that makes the platform convenient in order to get good performance.