What makes you say the Chinese did poorly against the Americans during the Korean war?

An under-equipped, under-supplied peasant army using bugles and flags forced a global superpower on the longest retreat of its entire military history (I think), and wore it down to what amounts to a neutral cease-fire, accomplishing the major strategic objectives of the Chinese state in the face of resistance against a vastly superior enemy force.

Seems to me they did pretty excellent.

There's a lot of factors, but one of the main ones was logistics. I wish I could elaborate more, but Chinese logistics was horrible. Infact, they even used that as an excuse to why some American POW's were malnourished. Also keep in mind it was only around 2 or so years since the end of the civil war. They didn't exactly have state of the art equipment either. What I can do for you though, is link you this short thing I just found, it sums up some issues with China and their military during the war: http://www.quartermaster.army.mil/oqmg/professional_bulletin/2004/autumn04/Initial_Communist_Chinese_Logistics_in_the_Korean_War.htm