Icon List block - Slowdown on Appearance > Widgets after revamp update

[andeng1106]

Describe the bug
Related to this issue: https://www.notion.so/Filip-b88ae8d8bd4841c48f3a3503cf56369d
Summary: User encounters a slow down on the widgets with a lot of icon list blocks after revamp update
Observations:

• Appearance > Widgets already loads slow when using 3.12.8 but loads slower on 3.12.10 update (see login credentials in the Notion page above for comparison)
• Tried out putting the new icon list blocks on a page, it loads fast as expected
• User has a lot of additional custom Widget Areas

Expected behavior

• Widgets Area should not load slow if there are a lot of Icon List blocks.

[bfintal]

PR: #3728