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To receive a high score, keeping the speed bonus (found at the top left corner over your score) is essential to beating your friends.
Once the speed bonus starts at +200, you have 1.5 seconds to make your next match.
Once you hit your first +1000 speed score, you have 2.5 seconds to make your next match.
This is extremely difficult to gauge, so try to aim for no lag time between matches.
You can start looking for your next match as you are completing the previous one or waiting for all the breaks to occur.
It is, however, safe to take a second to look for the best match.
Bejeweled Blitz seems to be programmed so that there is always a match on the board.
If you start having a hard time finding a match, start looking towards the top three rows.
Your matches will occur there.
Though, at that point, it is very likely that you should simply restart on a different board.