I think you should ask for the quickest 10 streak from the very start of the match as it takes spawn locks out of the answers. For me it was on Jungle as Janus when I rushed the MI6 spawn on the bridge and got 10 in just over 40 seconds and to 20 in less than 2 minutes.
I got an 18 streak to open a Heroes match on Memorial the other night and it couldn't have been much more than a minute. It was ridiculous. Just as the thought occurred to me that I had a shot at a thirty streak against this group I checked the score and got shot. The same thing happened once as Bond on Industrial. I spawned at the start and decided to check the score when we were up 118-4. Killed by the damned scoreboard.