loading up a saved game from CD: 15 seconds
loading up to get to the roof *from start of game*: 15 seconds
Loading up a saved game took 2.5 SECONDS, and same for coming off the Roof.
TOTAL 100 PERCENT GREEN ON DEAD RISING. WOAH
Yes, you can install one disc at a time.
You can actually install 2 games at a time, depending on how big they are.