Using dripstone to fill cauldrons.

Lava has only one level (empty/full).

Water has three levels (empty, 1 bottle, 2 bottles, full).

Tested and confirmed that caudrons fill with lava at three times the rate of water.

One would expect either the rates be approximately the same, or that lava be slower than water (Overworld, as it flows slower). One might also expect (untested) that lava fill cauldrons more quickly in the Nether (again, as it flows faster).

Even though bottles do not (presently) work with lava in cauldrons, I would suggest slowing the fill rate by maintaining three levels, as with water. (N.B. Whether bottles should work is a separate issue.)

I would also suggest the drip rate for lava be slowed in the Overworld, and make similar adjustments in the Nether and End, based upon relative surface-flow rates.