without knowing your setup I can't say this will work but here is how i have mine set up.
I have 5 Tier 5 skeletons spawners. I use a mystcraft portal but it easily be done with a mobavator, although a bit slower. The mystcraft portal drops them at the top of a drop chute. They drop down so they are 1 hit kills. At the bottom back of the drop chute I have a transposer set up on a timer. This sucks everything into a chest that goes through a sorter. Bones and Arrows go into barrels and everything else goes into a chest where the bows are sorted out with a diamond pipe into a void pipe, the rest(damaged armors) go into a diamond chest.
In front of the drop chute where I throw the healing potion in is a Brain in a Jar. This instantly sucks up all of the xp orbs.
I forgot I turned it on the other day when I was trying to get unbreaking 3 for my vile sword and after I got the book I threw the enchantment on and ran off to catch mobs. After I started getting unplayable lag, I headed back to my spawner area to drop stuff off when I noticed I had over 1k skeletons waiting to die. I turned off the spawner and gave it few seconds to drop all the mobs and threw the healing potion. After about 5 seconds all lag was gone and almost everything was sorted.
You just need to prepare your killing area and when something like this happens it isn't too hard to fix the problem. You definitely want to get a brain in a jar to suck up all of the xp orbs since this will be a major cause of lag when killing lots of mobs at once.