The bigger and bigger your configuration grows, the longer you have to wait for a "show run" output. This is because each time the show run command is executed, the router queries each interface to find the individual commands entered. To save you some wait time, Cisco introduced a command to cache your configuration in memory (for a measly 15K memory sacrifice):
Router(config)#parser config cache interface
Reduced the "show run" time on my 3640 router from 8 seconds down to 3. Nice! Keep in mind that you must execute the "show run" command once after issuing this in order to cache the configuration.
If you're interested, here's all the info: