I don't think it maintains all the memory of closed programs - the data/stack areas of memory will differ hugely between when a program is closed and when a pro

I don't think it maintains all the memory of closed programs - the data/stack areas of memory will differ hugely between when a program is closed and when a program is launched. If you did save all these areas of memory, the program's state would be exactly the same between opening/closing. However, the disk blocks containing the executable program file are likely to be cached in memory after you open a program. Memory is orders of magnitudes faster than the hard disk, so this would explain the speedup