Unless there’s something fundamentally wrong in the OS X graphics pipeline, it’s almost certainly CPU fill rate. With 4 times as many pixels as most applications are tested with, that’s going to be the component that gets stressed. The GPU will have no issues moving that many pixels, but it doesn’t help applications that rely on the CPU for rendering, which is most of them, including Sublime Text.
With regards to text rendering, Sublime Text already does glyph caching, Core Text isn’t in the critical path.