I was mining in a cavern, and I just thiught I had a lot of zombie mobs in the area. I eventually found the spawner, but I couldn't reach it without getting beat up. So I walled the area off and tried to tunnel through the wall and get it from above. I guess this allowed enough zombies to spawn and fill nearly the whole room. Everything was working fine, I had no lag, even with the massive mob... of mobs.
Well, I screwed up, and I fell in to the room. I respawned away from the place, so I ran back. When I got back to the cavern, I stopped at my storage room I built to craft a weapon, couple tools, and some torches. By now the lag was so bad thatt this simple process took over ten minutes. Sometime during this I did quit and restart.
Now what is wierd, is that while making the torches, a zombie ambushed me. When I died I noticed that I dropped way more gear than I was holding. On my way back to the store room, I killed the zombie, and it dropped nothing. When I got to the store room I found almost half the gear I originally dropped, including my damaged armour - except the leggings. After this, I went to the spawner - despite the lag - to break it. But, halfway between the store room and the spawner, the lag went away.
Before breaking the spawner, I ran back to the store room for a test. And the lag returned. Sorry for the novel, but I came accross multiple issues with just that one spawner. A spawner summoning too many mobs, a wall of lag, and a zombie that decided to bring me back half my gear, lol.
Try to recreate by dying near a spawner, loaded with gear, and with the mobs walled off. It appears to me the wall of lag is somewhere near the edge of the mobs render distance. Save and quit while in the wall of lag, and waita while to see if a mob brings back your stuff.
Well, even if that mob thing was a fluke, the wall of lag is still a bit of a nuisance.