Based on the fact he enjoys pain and has a high pain tolerance.Based on what?
Genjutsu , at least Tsukuyomi, are there to mess with your brain, not your body. Doesn't matter how hard and though he is in the Narutoverse, inside Tsukuyomi, Kakashi was made to look thin as a paper sheet. Inside a Genjutsu where user controls space and time, physical resistance means nothing, since it's all happening in your brain.Based on the fact he enjoys pain and has a high pain tolerance.