I mentioned to my sister that Japanese would take at least four times as long to learn as French (which she has already studied), and she asked me where that was from. A quick Google turned up a webpage of language learning difficulties for English speakers. If we assume that these difficulties are roughly symmetric, it goes some way to explaining why the Dutch and Scandinavians speak such irritatingly good English.
The slightly surprising thing is that Japanese is the only starred language in Category III: that is, according to the experience of the US State Department, Japanese is somewhat harder to learn than Chinese, Arabic, or Korean.
Or, in other words, Japanese is the most difficult major language in the world for English speakers.
I have to say that I do not find this implausible.