The slowdown was caused by the volume step calculation returning an
infinite value. This was caused by the calculation happening before
the object bounds are calculated via the code path which does some
early update for the displacement and hair transparency. The actual
value was never re-calculated after bounds are valid.
The solution is to only clear need-update after the final call of
the device_update_flags().
Pull Request: https://projects.blender.org/blender/blender/pulls/121042
b24c91e93e