This is the test setup:

Blueprint:
It should make 516 stone + 8613 molten copper per second from 6891 lava.
It makes 450 stone + 7500 molten copper per second from 6000 lava.
The foundry just gets stuck every so often, because its fluid capacity is too low:

It gets stuck when craft + productivity bars are about to reach 100% at the same time too.
It is not the inserter delay, because setup like this also cant produce over 450 stone/s:

With production rates so high I would expect the machine inventory size to be much higher.
It also seems impossible to use machines at 6000 fluid/s per input/output - highest I could achieve was 4500 for single and 7500 for double i/o - straight to/from infinity pipes. Adding storage tanks in between also didnt help.