We've done quite a bit of performance work, and there's a bit more in the pipeline, especially horizontal scrolling of very large lines. Some of the slowness at the latter is performance bugs in CoreText; we're considering more aggressively working around those. If you're still seeing slow performance, please file an issue.
alright, tried it out on a ~10K line file i have; lines can get pretty long, up to 1000 characters. xi is better than last time i checked -- it can open the file, and is snappy near the top. it slows down a lot near the bottom, which is where the longer lines are.
sublime, for comparison, is very snappy on the file.
i also tried it out on a pathological json file, which is just one line, 429984 characters long. it hung xi for a long time, and when it loaded, was very slow. again, sublime is completely snappy on the file.
probably not worth a bug since i think it's related to long-line support, which i think is already on your radar?
I think the Phi page is just including "current state of editor" as well as "aimed for state of editor" in the README.