Rendering to a static image that is replaced on initial interaction might solve the browser render time issue.
We’ve spent a month watching discourse run convert, optipng, etc to render optimized and smaller versions of uploaded image files from the G+ import…
I did this specifically in anticipation of @Ralph_Schaffner’s work. 
It’s just that until his work lands, STLs will show up as a download instead of being rendered inline with a onebox.
Update: @Ralph_Schaffner’s onebox handling for github STLs is in place upstream, so the next release that we update to should have interactive models inside oneboxes for STLs. It’s been a while since Discourse tagged a beta, and we normally update after they tag a beta.
@Ralph_Schaffner, are you still thinking of submitting a PR to Discourse to display uploaded STLs as models using threejs?
Yes, I’m still thinking about it. I haven’t decided on an approach yet. They appear to be trying to keep onebox generic so it can work with more than just discourse.
My first attempt kind of broke that separation.