Outline feature does not work with long page with many headings

[kingfungdesign]

Enabling the outline feature will cause the typing experience to drastically slow down in a page that has many togglable headings and more headings nested underneath the togglable heading.

To reproduce:

1. Create a page that has many togglable headings and headings nested underneath them.
2. The typing responsivness will keep decreasing as more headings are being added.

Browser: Microsoft Edge Beta