I thought it was pretty simple, but ok
I have cloud VDB loaded in corona volumegrid, it takes some time to start IR. After that I can move/rotate the camera and everything runs smoothly. But if I try to change the position/rotation/scale of corona volumegrid during IR it takes a lot of time. Same if I try to change some settings like scattering or absorption.
Just checked - 7 seconds for a particular VDB file. So if I try to move it or change the scattering scale I have to wait 7 seconds each time. And max is locked during those 7 seconds.
I can send VDB but I see no reason for that - it behaves the same with any VDB. The bigger grid (amount of cells) - the bigger lag.
Checked with Pixar cloud (I think you have it) - wdas_cloud_sixteenth.vdb runs smoothly, wdas_cloud_eighth.vdb a bit slower, lag is quite visible on wdas_cloud_quarter.vdb, wdas_cloud_half.vdb - 5 seconds to refresh IR, wdas_cloud.vdb - 13 seconds.