Switching between bookmarks folders is laggy

[the_goodone1]

Since Firefox 57, Switching between the bookmarks folders becomes laggy and not responsive like before.

Steps to reproduce:

1- make sure you have a busy browsing Session with lots of tabs open.

2- make sure you have many bookmarks folders on the bookmarks toolbar and many bookmarks folders with hundreds of bookmarks.

3- You'll find that switching between bookmarks folders becomes laggy.

I made a video to explain what I mean by "Switching between bookmarks folders". Unfortunately this video doesn't show the issue very well. https://streamable.com/xjjj1

this issue didn't happen with Firefox 56

I also reported it as a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1417438

I'm curious if anyone has also noticed this.