I'm not an admin, but have followed the sizing discussions around the lemmyverse as closely as I can from my position of lacking first-hand knowledge:

• lemmy.ml is the biggest instance by user count, but runs on incredibly modest 8-cpu hardware. Their cloud provider doesn't provide any easy scale up options for them, so they can't trivially restart on a bigger VM with their db and disk in place. I suspect this means that instance is going to suffer for a bit as they figure out what to do next.
• lemmy.world on the other hand was running on a box at least twice as big as lemmy.ml at last count, and I believe they can go quite a bit bigger if they need to.
• The lemmy.world admins also run mastodon.world and lived through the twitterpocalypse, seeing peak user registrations rates of 4k per hour. So this is not their first rodeo in terms of explosive growth, I'm sure that experience gives them some tricks up their sleeve.
• The admin team is pretty clearly technically strong. If I recall correctly, ruud is a professional database admin. One of the spooky parts of Lemmy performance-wise is the db. If ruud or others on the admin team custom-tuned their pg setup based on their own analysis of how/why it's slow, they may be getting more performance per CPU cycle than other instances running more stock configs or that are cargo-culting tweaks that aren't optimal for their setup without understanding what makes them work.

I'm surprised that sh.itjust.works isn't growing faster. They also have a hefty hardware setup and seemingly the technical admins to handle big user counts. I wonder if it's a branding problem, where lemmy.world sounds inviting and plausibly serious where sh.itjust.works sounds like clowntown even though it's run by a capable and serious team.