Ediging/Typing in the post/page editor is very slow in text mode

Hello folks,

I am having default theme (twenty sixteen) running over wp version 4.5.2. I’ve no plugins in active state (all deactivated). Having many pages created with long contents (about 500+ pages).

One of the page is having 18,997 words (https://surgemedia.staging.wpengine.com/6-mo-after-gastric-sleeve-warnings-successes). Please do not mind about the formatting since it is copied from live site to this staging site. When I try to edit content in this page in text mode, it is very very slow. I am not able to figure out what’s the reason since the content does not have any images or external content.

Please help!

Thank you, Naresh