I've GM'ed a few Pathfinder games, and while I really enjoy the setting, I just can't wrap my head around some of the crunchy mechanics. In particular, how do you (quickly) deal with effects like drain level in a pen and paper system - doesn't each player have to COMPLETELY recalculate their character sheet (feats, spells, skills, BAB) each frickin' time the mummy lord hits them? It seems like that would just grind the game to a smoking halt. How do other GM's handle this?
Negative Levels don't actually reduce your character level
They basically give you a -1 penalty on a bunch of things for each negative level you have.
For each negative level a creature has, it takes a cumulative –1 penalty on all ability checks, attack rolls, combat maneuver checks, Combat Maneuver Defense, saving throws, and skill checks. In addition, the creature reduces its current and total hit points by 5 for each negative level it possesses. The creature is also treated as one level lower for the purpose of level-dependent variables (such as spellcasting) for each negative level possessed. Spellcasters do not lose any prepared spells or slots as a result of negative levels. If a creature's negative levels equal or exceed its total Hit Dice, it dies.
So you don't actually have to recalculate the whole character, you just have to know what things are affected by level drain, and subtract accordingly. You don't lose any spells, or access to feats, or any skill ranks, etc.
So, really all you need to do is apply a penalty of (negative levels X -1) to all attack rolls, saving throws, and skill checks, and reduce hit points! Spell durations also go down and some other things, but that's not too hard to recalculate on the fly.
This is in contrast to older editions of the game when level drain did actually reduce your character level, so Paizo has baked in to the system a less-crunchy way to deal with the classic level drain problem.
It has been noted that in previous editions temporary negative levels could become actual character level loss. This has also been removed in Pathfinder (this section is right after the previous quote).
Some abilities and spells (such as raise dead) bestow permanent level drain on a creature. These are treated just like temporary negative levels, but they do not allow a new save each day to remove them. Level drain can be removed through spells like restoration. Permanent negative levels remain after a dead creature is restored to life.