One way to expedite small transactions would be to have all of the miner shun all further transactions by someone who has attempted a double spend, then if someone only wants to transfer less than 50% of their wealth then it can be assumed that it is a valid transaction because their expected return with a successful double spend is exactly what they would do with a valid transaction.
As far as I understand all that would be needed for this to be put into place would be to update the protocol, the blockchain can be preserved. Am I wrong? Is this a bad idea?