I encountered some severe performance issues after setting off large (power 50) fireball explosions in experimental snapshot 6. After having done so destroying or placing blocks or causing general updates will either occasionally cause major frame drops or consistently causes them in certain locations. It's likely not unique to these exact circumstances and is more a general performance issue but in certain circumstances this causes severe performance issues with as little as 4 pistons and 2 observers (debug screenshots included).

These issues were somewhat resolved when the game was restarted.