The fireworks seem to be coming out of disepeners/droppers under the water there. They never stopped for me. My nvidia driver version is 365.10 released on May 2, 2016[DOUBLEPOST=1462757354][/DOUBLEPOST]I was able to get the fireworks out of all but one dropper. Woo!
Were the fireworks being fired through water? I can't see it having much of an impact, but I've never seen that much issue with fireworks otherwise.
So, I've so far tried firing fireworks of several types through air and water, with no marked change in FPS. I have also fired them into blocks, both as quickly and as slowly as possible with redstone and dispensers. I do experience a little FPS loss when firing large volumes of fireworks at once, of around 10%, although this is nowhere near enough to be causing the issues described.
Could it be a combination of the 6 fireworks launchers all launching ... and mobs spawning in the rogue dungeon beneath?
You don't generally get that many mobs spawning, since it's an MyM server. I'll look into the possibility of multiple launchers now. It doesn't seem to have any particular impact. Are there any of these launcher still functioning in the world? I'd like to have a look at exactly what's going on with them.