In Chrom(e|ium), the font is loaded last and "blocks" the completion of the page loading for about 1 second.
Are you asking to remove all the icons provided by fontawesone to save that second?
Just curious: is this extra second for the first load of a page within a session or more, or for every page the user loads?
Even before looking at graphs, is there a problem of perceived slowness of Phabricator. I haven't heard any complaints about the speed of pages downloading, except for those related with the Maniphest reports (reported upstream, special case) and the Sprint app (resolved). In fact, I have heard a couple of comments about users happy because pages load fast. In my own experience, I don't think we have a problem at all here.
Upstream clearly will reject this proposal because they are increasingly invested in Fontawesome as a source for icons. Downstream we will not embark in the maintenance of such deviation from upstream, not even mention that we don't want to embark in any discussion with users surprised because the icons are now gone.
I propose to decline this task.
What's the actual proposal? Not using any logos at all (makes differentiation harder in some cases)?
Using icon (image) files that likely won't save a second either?
Don't browser have a cache for webfonts so they should only get downloaded the first time (in theory)?
It's a bit unclear to me what the proposal is. Simplify removing it does not make Phabricator a better experience for users but worse IMO.