Did you do a page load comparison between XF 2 loading the default fontawesome and your simplesvg?
Can we download the simplesvg script and host locally?
What XF template would you add the script in and remove the XF script? Also, we have to replace every FA icon that XF uses?
Its not possible because that depends on number of icons. However on average page it should be smaller. Script is 16kb + about 0.5kb per icon. Only icons used on page are loaded. Icons are loaded in bulk and cached, so on first page load it would load some stuff (in fraction of second), then icons will be retrieved from browser cache.
Yes. You can host everything yourself without relying on third party websites. API code is available on GitHub, SimpleSVG is configurable so you can easily point it to your API.
There are PHP and Node.js versions of icon hosting script that are very easy to setup and use. On simplesvg.com I'm running Node.js version because its slightly faster (by about 50ms) due to preloading all icons on startup.
That's tricky. XenForo 2 templates have hardcoded requirements for FontAwesome. Some of it is generated by template engine that can't be modified, stylesheet heavily uses pseudo elements. I've tried replacing it, after several attempts gave up because its way too integrated. XenForo 2 cannot be changed. XF2 code is way too opinionated to be changed, not to any other font, not to SVG. Then even if code is changed, third party add-ons rely on FontAwesome, so templates for add-ons must be changed too.
Replacing default icons in XenForo is not possible, but you can add more icons by including SimpleSVG script.
Are there any plans to upgrade to FA5 yet? I vastly prefer the FA5 icons over FA4 (e.g. the chat bubbles on the forums). Maybe offer an option to pick one of the 2? AFAIK they seem to be almost backwards compatible from the few checks I did (only difference being the design looks better IMHO).