Comparator update late when it reads a hopper minecart

When a hopper minecart on a detector rail is readed by a comparator the comperator needs some time to notice if there is a change of the number of items in the minecart.

What I expected to happen was...:
It should be notice it directly.

What actually happened was...:
It needs a long time until it notice a change.

Steps to Reproduce:
1. Put a detector rail on a block.
2. Put next to the rail a comparator that reads the rail.
3. Put on the rail a hopper minecart (or a chest minecart).
4. Put a item in the minecart and look at the comparator.
5. It is possible to update it faster by creating a block update to the comparator.