Performance has always been one of the most glaring issues with using Matrix. The article seems to be intentionally obfuscating the fact that performance and scalability issues stem from the activity a server participates in rather than just the number of users it has. If you're in a rather large room with hundreds of members and it's constantly active, you'll quickly notice your server struggling to keep up.
As a Matrix user, I feel like I'm being rug-pulled because Element HQ decided to gatekeep their solution to this problem that has plagued the community for more than half a decade.