Depends what you're writing. People aren't going to write performance-sensitive apps in Electron, like FPS games where customers demand 4K 120 FPS with full graphics enabled. But for Image burners (balena), Drone software (Betaflight), and Sophisticated code editors (VSCode), it's hardly a concern.
I use Discord all the time, which is one hell of an oranges-to-oranges comparison (with Slack), and not only does it have none of the perf problems of Slack, it's also faster and snappier than any NATIVE chat app I've used.
There's a lot that can be done wrong in any framework; as recently as windows xp, there were a few atrocious algorithmic-complexity gaffes in basic file viewing (directories with more than a few hundred files would slow to a crawl in the GUI, taking multiple minutes to display). It wasn't the fault of the framework it was written in; it's just that someone at MS had written something that had to scan the directory in a hurry, and ended up accidentally writing an exponential-time algorithm. Slack likely has a few deep-seated goofs in their architecture that are just too big to extract and fix.
Yes, JS can be much faster than every single piece of it you will find people using. It's just that every developer that has some main focus different from execution performance is holding it wrong.