For display purposes, UTF-8 vs. UTF-16 is going to be such a miniscule difference that it's not worth the potential portability bugs to try to optimize for speed. You're talking about at most 30000 characters of text on screen at once. If that's entirely stored in UTF-8, and entirely rendered in UTF-16, and the conversion takes an insane 100 cycles per character on average, you're still using less than 0.1% of a single core of a modern desktop CPU.

If you got into the 1%+ range, I could see justifying some attention to speed, but otherwise...