Soul Sand Elevator issue

I've built a 1 x 1 soul sand elevator, you know the one that brings you up? Well, i tried to make a system where the soul sand swaps with a magma block to get flow in both directions. down, until need to go up. When the swap from magma block to soul sand occurs, going up the elevator isn't a very smooth ride. there is some jitter that causes the way up to be very slow compared to a normal elevator. The interesting thing is, is that waiting 3 or so seconds before entering the elevator after the swap of blocks will prevent the jittering effect, which is still slower compared to getting in a normal elevator immediately. Pretty sure this is unintended.