Performance of displaying UTF-8 characters

Using a curses style game I wrote (github.com/gdamore/proxima5) - I noticed that there were big slow downs in iTerm2 (2.9.20151001 and also 2.1.4) when displaying characters outside of the basic ASCII character set. (I've noticed this in gomatrix too, btw -- github.com/gdamore/gomatix .) I can send you binaries to try if you like.

What is very unusual about this is that it seems to depend significantly on the font used. Menlo seems to perform much better than Consolas or Andale Mono, for example. (All at 14pt.)

Furthermore, I tested this with Terminal, and also using the same different font settings, and was unable to notice a difference. So I guess something about the way iTerm2 deals with fonts is causing ugly slow downs when using unusual characters.

I believe I saw this on Yosemite with gomatrix, but I'm using El Capitan now, so its a current problem.

(Note that I spent a bunch of time trying different things believing that at first it was something bad my program was doing -- inefficient screen redraws or somesuch. But the fact that changing the font made such a big difference seemed to me to exonerate my program.)

This isn't critical for me, but I'd love it if you could look into this. For now I'll just use the Menlo font. (For most day to day work, like vi, etc. it doesn't matter. But when working on my game stuff, I need a reasonably decent display performance.)