Where did you get the measurements from?
Depending on the size of your "hit" documents, search time varies. 50ms is consider very good.
Caching is one of the reasons that you might see a very fast result occasionally (ie issue the same search very quickly back to back).
7ms to 50ms is a bad comparison/reference IMO.
I would increase the range of your search to include more documents so your search time is in the hundreds of MS to start with and compare.