I traced this down and it's a direct result of RenderText taking a long time to figure out the width of a Japanese character. But this only happens the first time RenderText has to do something with a Japanese charset. For example, if I open a Japanese website first, the menu will open quickly. I would try to defer this slow process, but it's always going to be slow. I can see a similar hiccup when I start typing in the omnibox and one of the suggestions uses Japanese characters, if it's the first time we've had to load Japanese in this session.

I wonder if this is on anyone's radar or if there's anything we can do about it generally. Can we pre-emptively load charsets that are likely to be shown at some point on a non-UI thread?