Well, here's the basic idea: when you have your data in the system partition, the read/write head only has to move a small distance between where the programs are and where the data is. If you have the drive split in to 2 equal partitions, the drive head has to move halfway across the drive in order to read data after loading programs.
As to "reading only": your OS downloads updates regularly, and programs modify the registry.
However, the one true test is what happens in the real world. My work laptop used to be configured in a 2-partition scheme; when I replaced the hard drive, I got rid of the 2-partition scheme and went to a single partition... I didn't really notice a difference.