my guess it that since the water need to flow through all of the tanks and all of those tube one after each other, that your drain rate is significantly reduced.
I assume that the pump runs from the lowest point to the highest.
Can you tell me this, when you turn off the return pipe, and put in a cup or two of water, does it come out into the sump? How long does it take for the water to start coming out of the bottom, and how long to stop. This might give us some incite as top your flow rate.