I've now experienced this on multiple pages, but attempting to edit an entire article with many tables makes 2017 WTE react really badly. Barack Obama is a breeze in comparison, so it's not just pure page size, I don't think. This is in Firefox 57 on Windows 7.
I had to turn it off to make this edit today.
It might be due to having find and replace open in specific, or possibly with the fostered content which was the point of the edit, since I was just able to make a followup error fix pretty easily.
The editor doesn't attempt to parse the wikitext in any way (unless you are using the syntax highlighting beta feature) so an article with tables will load as quickly as one without. It could be related to find and replace as you say - but that only runs when you open it.
I managed to make it happen just by selecting a lot of the text in another article with a number of tables and then hitting ctrl+v (don't know what was actually on the clipboard). Bah.
The team made a lot of performance improvements to the 2017 wikitext editor after this bug was filed a while ago, and the page seems to load really smoothly for me now, so I think the fundamental issue here has been resolved.