I've been building in the same world using just a few mods with no issue for a few months now, but suddenly tonight, in the middle of building, my frame rate suddenly fell to around 2fps.
I've tried closing and reloading the map, and it loads never going above 2fps. I've tried recreating the same seed and loaded that world with it working fine. I've also created a new world and had that run that without any frame-rate issues, so whatever the issue is it's specific to the save for this one world, and has nothing to do with my hardware.
The mods I'm running are Forge 1.11, X-ray, Minions, and Journey map.
Is there any way to hard-load my world while at the same time preserving anything I've built? I think that might fix whatever issue I'm having.
Something you could try is to re-generate the world, and then use World Edit to copy/paste your builds into the re-created world to see if the issue still happens.
The FPS issues could be with any of the mods, or it could be because of Minecraft Forge. Minecraft Forge for 1.11.2 is still in beta, and stuff is bound to break. Heck, some mods don't work at all with the recent builds of Minecraft Forge for 1.11.2 but will work with older ones.