Federation takes quite a toll.

The way I read the article is that they didn't upgrade every time the load was (almost) too high, but instead opted immediately for the beefy dedicated server (considering the price it makes sense – it's on par with beefy VMs that have less power). They probably have resources to spare right now, but no need to go through the hassle of an upgrade soon (although 4 mins downtime sounds like they have things under control).

Having said that, I'm sure there's plenty to improve in Lemmy, given it's used at a bigger scale now.