The usual computation cycle is
storage -> ram -> register -> cache -> ram -> storage
it changes to
storage -> register -> cache -> storage
so there will be more instructions since we don't use the RAM.
as an example with a fictional CPU just to get the difference impact
1 Cycle is 1 Hertz (obviously)
-> l1 cache takes ~16 cycles
-> l2 cache takes ~32 cycles
-> l3 cache takes ~48 cycles
-> ram takes ~256 cycles
-> hd takes ~5012 cycles
so accessing things directly from the HD should be small because they are rather slow in comparison.
but if you got limited RAM for example and enough small programs this can be a good solution or if time is not really relevant for certain processes.
The usual idea (obviously) is that "preloading" all the information into the RAM to increase the overall performance. But maybe you don't need it at all and in the end you even save cycles by not using the RAM as buffer.
It's as so often -> time vs space and in this case space is critical so we trade against time.
Edit:
i forgot one classic / important use-case -> isolation of a program if you use an usb-stick for example you can "trap" the program inside the Stick and just use the CPU of the computer