Some host throttles the requests and for every change you made, it issues requests to the server. That includes post request, preview request, and so on. Some also throttles the CPU usage in which affects the requests and can get slower.
The amount of changes and editing you're doing could trigger that issue which unfortunately, the builder or the theme has no control. If your host do that, then I recommend doing the editing outside the text element (like notepad or any text editing application) and just copy and paste the text. With this, it minimizes the requests of the current page within the builder.