If you were writing pure JS you'd probably need to intentionally delay text entry with setTimeout/etc to achieve this effect - I don't think you can achieve this outcome by accident.
The problem is that the front-end ecosystem encourages pulling in megabytes of third-party crap for every little thing (including poorly reinventing existing browser functionality) and the result is that some interaction between all that mess causes an entire page redraw upon every keystroke or similar.
To be fair to authors of those third-party libraries, some of the blame has to do with browser quirks, and for a rich text editor, bugs in the implementation of content-editable that have to be normalized across all browsers (unless you pull the engineering-heavy route of rewriting everything in canvas).
The problem is that the front-end ecosystem encourages pulling in megabytes of third-party crap for every little thing (including poorly reinventing existing browser functionality) and the result is that some interaction between all that mess causes an entire page redraw upon every keystroke or similar.