I don't really understand the hacker love for IRC. It's always been hard to figure out what level of encryption you can expect, what plugins are supported on what clients, using it from multiple devices was never a good experience, you miss any messages sent while your pc was off, very poor support for searching server history for messages with certain text, sending files can be confusing, etc. Having a snappy client is cool but it'd be cooler if it supported half the modern features people enjoy when using chat.
The native client they were referring to was mIRC, which does much less than Slack does. A better comparison might be Discord, which performs much better than Slack and is also written in Electron. VSCode, written in Electron, performs much better for me than Visual Studio (a native app), but I wouldn't use that as proof that Electron is faster than native apps because Visual Studio does so much more
The major differences I can think of off the top of my head are threading, reactions, voice chat, and history. Voice chat is the only one that should use significantly more resources than mIRC.
I'm not really comparing the slack of today with mIRC which was first released in 1995. In early 2000s back when IRC was still very popular, mIRC was a breeze to use on computers which existed. It hasn't seen much development over the past decade and it shows, but it's commendable that lightness was a priority for developers who built it.
