From experimental snapshot 2 changelog:

• "Reduced the chance of sand and gravel being placed in such a way that they immediately fall down on generation. Replaced with sandstone or stone in those cases. The falling blocks were messing up the terrain as well as performance. Still happens but not as as often."

This rule does not seem to apply to naturally generated lava pools, instead - the sand just falls down now.

Note that before experimental snapshots the sand would have stood floating instead of falling down, indicating some sort of change. This sandfall causes a lagspike.

The solution would be to apply the same sandstone placement rules to lava lakes, or revert it to pre-exp. snapshots to make the sand float above the lava lakes again. 

Seed: 9094103481074268094

Coordinates: -3463 69 -902