It all really depended on where you were when the volcano detonated. If you had the chunks it was in loaded at all, you'd likely begin experiencing massive lag due to the mass amount of chunk updates and lighting updates, as well as sound if you're closer.
Either way, we got enough reports and first-hand experience ourselves that it had to be removed.
The lava was one block high, but even with only one block high, that's still a mass amount of blocks to cause sound lag (because of Minecraft's sound engine) and a large amount of chunk updates. The point was to give people a chance to live by making it flow down like an actual volcano, not to instantly murder everyone in the segment by filling it with lava.