Extreme lag/game freeze around lighting glitches

Primary Problem: Encountered dark spots under mineshafts hanging mid-air in SMP, game appeared to freeze for a few seconds. Correct shadows would be shown when game resumed. Vulnerable to hostile mobs while game was "frozen."

Full description: I was playing on SMP running a sand flatland world with mineshafts enabled. Underneath the mineshafts I frequently encountered lighting glitches (dark spots) where many hostile mobs had spawned. Once I got close enough, the lighting would be updated but this typically caused a near freeze in game performance. After a few seconds, the game would resume at normal FPS with the correct shadows displayed. The frustrating part is hostile mobs seemed to be unaffected so that when normal gameplay resumed, I would be surrounded by mobs and already taken a few hits of damage. Attempting to run away resulted in running into more glitches/more game-freezes and I would die.