More RAM. Forget upgrading the CPU. Usually upgrading laptop CPUs is a major operation. Upgrading RAM can be as simple as opening the 'RAM' hatch and plugging a suitable module.
The machine is probably getting 'laggy' as your application uses more and more memory and starts 'paging' to disk. (Swapping currently unused bits of RAM to disk so it can is it for something else).
Laptop hard disks are notoriously slow. You DON'T want to start 'swapping' to disk.
I've got a Dell D420 with a core duo CPU that is normally quite snappy. Until I tell 'JAlbum' (A JAVA photo web site generator) to process 100 photos. Machine gets terribly slow. JAlbum/JAVA uses more RAM than the machine has physically installed, so the operating system starts swapping memory pages to that super slow miniature low power laptop hard-disk.