Fast switching makes the routing of packets more efficient by caching the next hop information of packets to a specific destination, so subsequent packets don’t have to go through the lookup process to determine what the next hop is going to be.
Fast switching accomplishes this by processing the first packet received to an unknown destination with Process switching. When process switching the first packet, this allows Fast switching to save the next hop information by caching the next hop information in the CPU. Cache entries are timed out quickly to prevent the CPU from being overloaded with cache information for multiple destinations. Likewise the initial process switched lookup can also be cpu intensive if there are an influx of packets destined for addresses not found in cache. At the time of its inception Fast switching was an enormous improvement to the process switched problem.