XenForo uses Font Awesome Pro 5 which contains several thousand icons but where XenForo usually only uses a few hundred icons.
By removing unused icons, the size of the CSS file can be reduced and this improves page load times.
Usually, the size of the font css can be reduced by more than 50%, depending on the number of styles and addons you are using.
The XenVn add-on's Optimize Font Awesome feature will scan your system for all Font Awesome code in use. From there, create an optimal CSS code for Font Awesome.
Here is an example image when turning off and on the Font Awesome optimization feature:
Enable Font Awesome optimization:
The feature is completely automatic, you just need to turn it on in the settings of XenVn.
Note: When you change any Font Awesome code, run [XenVn] Rebuild FontAwesome in "Tools -> Rebuild caches" to update Font Awesome code data.
This is a feature part of XenVn addon, see details here: