That would be because with a new user account you get a clean slate of startup items. The only reason your mac will slow down at startup is because of your startup items and processes. The reason your main user account takes 5 to 10 seconds is because you have more startup items. To change this go in to your System Preferences and go to accounts and go to the Startup Items tab. Remove any unnecessary items.
As for other users that are noting startup slowdown, same deal. It has to do with how many programs and processes are being run when the system starts up.
Macs are designed to run different maintenance scripts daily, weekly, and monthly which will help keep things running smoothly however these scripts are set to run at times when your not likely to be using your computer (so you don't notice the slowdown while they run), usually 3am. If your computer is off or in sleep they won't run. A great program to use to run these scripts manually is MainMenu. But as with all programs that let you access system information be careful with how you use it.
Finally, as we all know with Windows, the more you download and install the slower things get, same goes with Mac (though not nearly as bad). If you install tonnes of stuff and never remove it your HD space gets eaten up and when that gets low, things will start to slow down. It's recommended to always have a couple of Gigs free at all times on the HD the OS is installed on.
