Laptop vs Desktop for processing

I have noticed that my desktop which is older and less powerful than my laptop in every way other than the hard drive speed, is able to process images, such as focus stacks much faster than my laptop.

My desktop is a DDR1 4 gig 32 bit dual core at 3.10ghz with 3 7500rpm hard drives, photoshop and pictures saved on different drives
and laptop is 64bit core 2 duo 2.10ghz DDR3 4gigs with a 5400rpm hd, everything runs off of one drive.

I have been thinking about upgrading to 8gb RAM but when I ran a 21 image focus stack of 21 RAW 18 megapixel images in CS5 on my desktop the highest the RAM peaked was 2.5 GB and it took about 4 or 5 minutes. If I did this same task on my laptop it would take 30 minutes. Does hard drive speed play that big of a role with loading images into photoshop? Do any of you have a similar setup at home with this sort of performance comparison? If it is about hardrive speed then I am going to get an SSD harddrive instead of a RAM upgrade unless anyone can recommend otherwise. thanks

I wonder if your graphics processor comes in to play or not. Maybe the video card in you desktop is better than the one in you laptop. Also your desktop is 3.1 ghz whereas your laptop is 2.1ghz - maybe it is the ghz that is relevant.

Have you tried downloading a benchmark program and running it on both computers to see how their score compare. In my experience desktops are nearly always more powerful than laptops even when they are a year or 2 older.