sigh JS is imperfect but it doesn't just make everything it touches slow. I would be very surprised if in Discord for example any JS was blocking between you pressing a key and the key appearing in your text box.
In fact I took the time just now to run a few samples in Discord and it's looking like I'm getting the same 35-40ms keypress to photon latency as gVim with one 80ms outlier out of the 5 runs I did.
The difference is very noticeable on a less-than-new CPU (my x240 with an i7 4600U for example) when the text input triggers other UI changes. Even on my Ryzen 3600, switching channels in Discord has visible lag, regardless of network activity.
Compare this to any operation in Sublime Text/Merge and the difference is obvious.
The implication is also that more battery power is used for every action (not to mention vastly more memory), which matters for machines of any capability; but these are not latency-related concerns.
In fact I took the time just now to run a few samples in Discord and it's looking like I'm getting the same 35-40ms keypress to photon latency as gVim with one 80ms outlier out of the 5 runs I did.