Oceans spawn thousands upon thousands of mobs and drop my framerate from a steady 300-400 to less than 10 whenever I'm near an ocean.
Also, I get no mob spawns anywhere else in the overworld. I've seen one Scorcher spawn in the Nether but that's after several thousands of blocks of searching on a test world I made.
Best advice I've seen is to lower render distance but obviously that only applies for the ocean issue and does nothing to help overworld mob spawns.