Forks absorb the braking force of you and your bike, while transmitting critical feel from the front end to the rider.  Stock forks are almost always undersprung, and even those lucky enough to  have proper OEM spring rates for  their body weight will find the rebound and compression valving woefully inadequate for hard riding.  Whether all you can afford is springs and valves, or you're ready to step up to the perfomance advantages of drop in cartridges, improving your front end from stock is an absolute must if you want to go faster.