We spent one of our ten day trip to Shanghai at the Expo. The area was huge and for the bigger and interesting Pavilions, like the German or the UK Pavilion, the waiting time was between 4 and 5 hours! Nobody wants to stand that long in a line on a very hot summer day.
It was so hot, that some places had installed water steams coming out of the ground. That was a nice cooling and made the walking easier.