Now that you have installed OS X on your USB flash drive, you’ve probably noticed how slow it seems. This is normal for flash-based drives, and there’s not much you can do about it, except to buy the fastest USB flash drive in your price range.
It’s important to remember why you’re creating this startup drive. It’s for use in an emergency, when your Mac won’t boot, either because of a hard drive problem or a software-related problem. A bootable USB flash drive will help you get your Mac back to working condition, by letting you use all of the tools a fully-working Mac has available.
In addition to being able to use Disk Utility, the Finder, and Terminal, and have access to the Internet, you can also load some specific emergency tools onto your USB flash drive. Here are some of the utilities I suggest installing. You don’t need to have all of them; in fact, it’s unlikely they would all fit onto the flash drive after you install OS X, but having one or two certainly makes sense.