Beach-balling on macOS when adding large RSS feed sources

[stephendonner]

Description

Beach-balling on macOS when adding large RSS feed sources

Steps to Reproduce

1. install 1.38.23
2. launch Brave
3. open a new-tab page
4. click on Customize at the bottom right
5. click on Brave News
6. enable Brave News
7. under Your Sources, enter https://planet.mozilla.org/rss10.xml
8. click Add source
9. wait / "beach ball" a bit
10. click to (try to) focus the Feed Url textfield
11. wait / "beach ball" a bit
12. enter https://planet.mozilla.org/rss20.xml
13. click Add source

Actual result:

NOTE: Sadly, the beach balls don't show up on this screencast. So, instead, you can picture them appearing in the pauses between the last Add source success and me clicking on the textfield to focus it (and trying to paste the URL until I'm able).

Expected result:

No "beach balls", or at least minimal.

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.38.23 Chromium: 99.0.4844.51 (Official Build) nightly (x86_64)
Revision	d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS	macOS Version 12.3 (Build 21E5227a)

cc @petemill @mattmcalister @kjozwiak @srirambv

[fallaciousreasoning]

Duplicates #23909 (which is fixed)

[stephendonner]

Duplicates #23909 (which is fixed)

Thanks for fixing!