Hoppers will be slower to pull items when there are multiple minecarts to pull from. It seems that they will only attempt to pull from one minecart even if the minecart is empty or otherwise cannot be pulled from. A similar bug may occur with loading of minecarts, but I haven't tested it.

To reproduce:

1. Set up two hoppers facing into a chest with rails on top of them.
2. Place two chest minecarts above the left hopper such that the hopper can pull from both of them (see rep-1.png for an example setup) and one on the right hopper.
3. With the hoppers locked, put a stack of stone in one of the minecarts on the left and a stack of dirt in the minecart on the right.
4. Unlock the hoppers and look in the chest. It will fill up with dirt more quickly than it does with stone.