Large Storage (Mk3/MK4) can Send faster than it can Receive

I've got a mountain I'm removing that leads to most trucks having 1/2 dirt needing an Ore Sorter
I have 8x T3 Belts to 2x large storage, and another 2 for dirt

The Large Storage will alternate input belts and leaves some stuttering despite plenty of room in the storage and outputs.

Testing was done 4x T1 storages inputting 1 belt each into 1x T3 Storage.
Output from T3 through a balancer (Strictly Even Output) back into the T1 storages.
Force a buffer (or deliver a load) into the T3 storage.

Expected behaviour is to maintain the same level in the T3 Storage (Input = Output)
Actual behaviour, the T3 Storage will slowly empty indicating faster output than input, and gaps will appear in the output belts.

Pausing 1 output belt will very slowly increase the buffer within the T3 Storage which indicates the input speed is >1350 and <1800

There is a limit on the throughput of a storage, is that what this is about?

not quite, the Input throughput is less than the Output throughput
As it stands, the 4 belts can't pump in items at their speeds, because the storage wont accept it, i.e. the Storage Throughput is lower than the Belts

I would also expect the throughput of a storage to be equal to the top tier belt/pipe multiplied by the input ports.
T1/T2 storage, 450 * 2 = 900 items/s.
T3/T4 storage, 450 * 4 = 1800 items/s
T3/T4 tank, 900 * 4 = 3600 fluid/s