After you've placed the four end crystals around the empty bedrock portal to make the Ender Dragon respawn in The End, the end crystals don't immediately respawn after the explosions that occur on top of the pillars.

However, this might be intentional because of the reason that the player can destroy the ender crystals before the actual Ender Dragon spawns. I know that this is a bug report site, but maybe the dragon could respawn instantly after you've placed the four end crystals?