@hrefna In my experience running small fedi instances from the garage in my back yard for the past 7 years I would say the problem is real if a bit overstated. A consumer grade gigabit fibre service handles this use case surprisingly well.
That said, Mastodon especially but fedi apps in general are quite inefficiently designed, and the single biggest challenge probably is the preview card issue. There are straight forward ways to mitigate the "fedi-hug" problem (which is actually not anything to do with ActivityPub) at the application level. Preview cards could be fetched "on demand" (so could post threads from origin servers, which would be nice and servers could just, you know, only fetch preview cards only for local posts then federate the data amongst themselves instead of flattening the poor potato servers serving the links we share.
Nonetheless I figure that at comparable scales and levels of distribution that ActivityPub is potentially quite a bit more efficient on resources than AT protocol.