This is because the MacDraw II file has to be converted to MacDraw Pro format through the MacDraw II 1.1 Translator. The conversion process takes more time than opening a MacDraw Pro document, or opening a MacDraw II document in MacDraw II.
This conversion process includes converting all the colors and color patterns to similar solid colors in MacDraw Pro. Also, smoothed polygons in the MacDraw II document are converted to bezigons in MacDraw Pro.
A MacDraw II drawing with thousands of objects may actually take an hour or more to open in MacDraw Pro.