[buboard]

Well my question is why they consume so much memory/CPU? What makes these services so different from earlier chat systems? Are these users continuously doing video chat with each other or keep sockets open? For simple text chat, 4GB of RAM seems absurdly high, considering how irc was able to handle thousands of users 3 decades ago.

[price]

IRC is essentially stateless. The server doesn't remember what messages people have sent; so it can't tell you what was said last night when you come online in the morning, or what was just said in some channel you weren't previously listening to. That work gets pushed out to clients, and to add-on services.

The IRC server also doesn't store images or any other kind of file people want to show each other. There are lots of good practical reasons to want to share images in a conversation (e.g., screenshots), plus of course silly GIFs. That work also gets pushed out to add-on services.

When people say here that Slack or Zulip etc. are a much better user experience than IRC, I think those two things -- message history, and images -- are major reasons for that.

Message history means a database that gets big, and images mean a lot of data too. There's a large working set of both of those that you want fast access to. That means providing adequate RAM.