Cache when windows minimized

When I minimize a window it appears that the program or
the open files are cached to the hard drive. Minimizing
something like photoshop with a large file can take ten
seconds or more with the keyboard locked out while the
hard drive runs. It doesn't happen when Restore Down is
used to make the window smaller. Is this a fixed
condition or can it be made to cache to RAM instead of the
hard drive?

Thank you,

Norm