Korean Madness (48 or 53)
 

It goes at a relentless 175bpm all the way through, requiring (at least for me) significantly more stamina than any other song below the 50's. The section around the 410 mark is very dense and hard to read, especially with the chart's custom background.

Re: Korean Madness (48 or 53)
 

AFAIK, backgrounds arent a factor in difficulty ratings because you can disable them.