Elasticsearch - EdgeNgram + highlight + term_vector = bad highlights

Hello,

I've originally posted this question to StackOverflow but nobody answers
so...

When i use an analyzer with edgengram (min=3, max=7, front) +
term_vector=with_positions_offsets

With document having text = "CouchDB"

When i search for "couc"

My highlight is on "cou" and not "couc"

It seems my highlight is only on the minimum matching token "cou" while i
would expect to be on the whole word or at least the longest token found.

It works fine without analyzing the text with
term_vector=with_positions_offsets -> the highlight is on "couc" and not
"cou"

What's the impact of removing the term_vector=with_positions_offsets for
perfomances?

Thanks