Somewhere in gnembon's Simple Hostile Mob Farms, Minecraft 1.12 (Fun Farms Ep. 15) video he mentions difficulty and local difficulty but I cannot remember where in the video.
In my 1.12 survival world I ran my build of his farm for an hour, getting about 4600 items/hr on Normal and about 6500 items/hr on Hard (regional difficulty 3.19, which is much lower than the max of 6.75). I ran the Normal test before the Hard test so the local difficulty may have changed slightly. Some of this effect might be due to pack spawning or just higher spawn rates.
The answers from Advantages of hard mode in minecraft are outdated.
I am looking for evidence based answers (tests, looking through game code), not hearsay or what the wiki says if it is unsourced.
Best Answer
Yes, hostile mobs will spawn more when on hard mode than easy and normal mode.