Prefix queries are expensive here. The problem is
that each one forms a very large OR clause on all
the terms that start with those two letters. For instance,
if a field in your index contained
mine
milanta
mica

a prefix search on "mi" would form
mine OR milanta OR mica.

Doing this across seven fields could get expensive.

Two things:
1> what is the problem you are trying to solve? Perhaps some
of the folks on the list can give you some suggestions. You can
think about many strategies depending upon what you want
to accomplish. A 300M index isn't very big, so you could, for
instance, think about indexing a separate field that contains only
the two beginning letters and search *that* in this case. I'll
assume that three letter prefix queries are OK.

2> How are you measuring query time? If you're measuring the
time it takes when you first start a searcher, be aware that the
first few queries are usually slow because the caches haven't
been filled. Further, are you measuring total response time or
are you measuring *just* the query time? It's possible that the
time is being spent assembling the response in your code
rather than actual searching. You might insert some timers
to determine that.
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This is taken from the lucene mailing.

HTH