This could be a number of factors. For example what is the spin speed of the new HDD and what level cache has it got? Spin speeds could be typically 4200 or 5400 and caches 4MB/8MB. Compare these to your old HDD.
Also, depending on your laptop,check in the BIOS for options to use accelerators for the HDD. Perhaps if you post back the make and model someone will be able to help. You seem to have plenty of RAM anyway!