That's because the Lemmy webapp focuses on being lean and functional rather than shoving as much telemetry and megabytes of JavaScript bloat as they can to do LESS than the old Reddit webapp could while using 10-20 times MORE resources.

New Reddit is not completely unusable on an intermittent, crammed full 3G connection where Old Reddit just works, but is known to be actively user hostile and somehow cramming full a huge 1080p phone display with only 2 or 3 comments and having to preload for hours for a full thread.

Lemmy server is also blazing fast and being written in Rust which encourages memory safety does help it function better on smaller instances and serve both local and federated clients faster despite having less resources to do so. I really really hope this replaces Reddit in the mainstream and people learn basic concepts about federated media to future proof the free Internet.

It has been a real breath of fresh air and so far it seems more sustainable to spread the bandwidth between smaller instances than let a megacorp fund the infrastructure to serve everyone in a walled garden which will later be enshittified into garbage once a critical mass is already lured in.

EDIT: look for yourself and notice the difference on Firefox's dev console network tab