(Note: I'm of the opinion that fediverse-style federation in the context of forums is merely a nice-to-have; the web is already naturally federated, and people should not feel bad if they want to save money/tech complexity/administration complexity by settling for ordinary self-hosted forums.)
Is there a reason for this, though? Need to be able to iterate on features quickly? Maybe not being able to tackle various complexities with the total available resources? Or maybe federation is just inherently expensive?
Why couldn't we have an alternative written in a more performant language/runtime with maybe things like lower quality images/videos or something?
Because performance was not a concern when it was designed - or it could be that it was designed for small communities, and therefore not possible to scale-up cheaply. One of the problem is the caching of pictures from the different instances connected (if I remember correctly) which makes the data storage requirements go up very fast