Swapping themes causes a long client freeze

[Crystalwarrior]

Describe the bug When pressing "reload theme" or receiving a subtheme from the server, there's a few seconds of lag before the user can use the client again.

To reproduce Steps to reproduce the behavior:

1. Go to a server
2. Click on reload theme
3. It lags

Expected behavior There is little or no lag and the theme swapping is optimized

Version:

• OS: Windows 10
• Version or branch: master

Additional context I remember optimizing some of the lag before, but it got pretty bad again on master. We need a solution pretty bad especially since sub themes have a reload theme request