Empty minecarts and storage minecarts currently lose 4% of their momentum every tick, or 56% every second. They've always done this, but before 1.6 it was easy enough to compensate by using a stronger booster. However, powered rails always apply a constant acceleration of 1.2 m/s/tick.

With this level of drag, an empty minecart can never move at maximum speed for more than 1.6 seconds after leaving a stretch of powered rails of any length (and it will stop completely after a few more seconds). This makes powered rails impractical for boosting empty carts and storage carts, unless you're in creative as one powered rail is required every 4 blocks to maintain maximum speed. (compare to 1 every 36 for players)

The only alternatives I've found for empty cart propulsion are powered carts (which are very slow and must be fueled manually) and spin boosters (which are very unreliable and likely to be removed).

PS. I would have made this issue an Improvement, but the issue creation form won't let me select any type other than Bug.