I haven't measured it, but it feels like 1-2 ms per keystroke.
Anyone else experiencing this? I am using a USB keyboard on a Mac as the Bluetooth keyboard was also giving a minor lag (but likely less than 1 ms).
Right now it is driving me bonkers.
To answer your question though: I find Slack's typing responsiveness acceptable.
EDIT: To narrow down the source of your problem you might try to use Slack in the app if you are using it in the browser or vice versa.
My usual issues with Electron apps are about performance in general, not (just) input. There's a good chance the browser version runs better, and afaik it should also have a smaller resource overhead.