XenForo uses the icoon font Font Awesome Pro 5 which contains a few thousand icons of which XenForo does usually only use a few hundred.
But as all icons are included in the CSS and font files, browsers have to download the full font files and CSS which does cause a significant overhead in most cases.

By subsetting the font to include only those icons that are actually being used, the size of the font files and CSS can often be drastically reduced which does improve page loading time.
Typically, the size of the font files can be reduced by over 80%.

This Add-on does allow to automate the process of creating subsets as much as possible:

• It can automatically scan the database and PHP files for Font Awesome icon references
• It can monitor database entities (like Phrases, Templates, BB-Codes, etc.) for changes and adjust the subset accordingly
• It can perform subsetting using locally installed fonttools/pyftsubset or transfonter.org
• It does allow you to easily add icons that are not discovered automatically
• It does allow you to exclude icons that have been discovered automatically but are not actually being used (for example if the feature that uses them is not being used)
• It can exclude certain templates or files from being scanned (which can be useful if those reference icons without actually using them)
• It does offer various strategies to load subsets and CSS
• It does support AMPXF

If pyftsubset is not available, this Add-on does use transfonter.org to create the subsets, eg. it does make calls to this service during installation and normal operation.