Normally while playing my frame-rates are totally fine (60+), but when flying near large pools of lava, especially in the nether, I experience massive amounts of FPS loss and freezing. This happens both with and without any client mods (I've tried vanilla, with MCPatcher, with Optifine - nothing makes a difference). The debug window also shows sound shooting up massively during the FPS loss.

A few others on my server have confirmed having this issue too, and there does not seem to be any particular common denominator that's out of the ordinary.

The freezing and fps issues stop when you disable sounds, so I can only assume it's due to the massive amount of popping and lava-ish sounds happening all at once.

I attempted to try it just without the lava sounds in particular, but Minecraft seems to be smart enough to just re-download the sounds when i remove or overwrite them!

Some looking around also led me to forum posts with others who are having the same issues, it seems to be fairly wide-spread and definitely involves lava and sound.