Comparators measuring a container take redstone 2 ticks to change their state instead of 1

Comparators send their signal too slowly when activated. This causes a problem with unloaders because the powered rail doesn't deactivate in time to stop the minecart chest over the hopper. Usually the minecart bounces off the block and returns back along the track. The hopper is only has time to disgorge one item from the cart per run.

I've tried a couple of designs and both have the same issue.