Although the Android OS is designed for multitasking, the G1's limited internal memory (128 MB) makes it very difficult to run more than one or two apps simultaneously. Considering that the OS itself takes around 30MB of memory, the phone is left with abysmal 98MB of free RAM available. Even less if you have many apps, since they all get installed to the same internal memory. Each app taking anywhere from 500Kb to several megabytes, you can see that the RAM will get eaten up very quickly as you pile on more applications.

The OS was therefore designed to free up RAM for currently active application by killing off apps running in the background. Sometimes the Home application itself gets killed off in this process, and needs to reload once you exit out of another app. That is why you are seeing a blank screen for a few seconds.

If you are running the Android version that shipped with your phone (a.k.a. "Stock" ROM), the maintenance steps suggested by Slinky1914 will help a little. Clearing Browser, Maps, Email and Market caches (that are yet again stored on the phone's internal memory), not using many widgets and icons on the Home screens to lighten up that app, uninstalling apps you don't use or rarely use, exiting apps instead of switching from them, etc.

Unfortunately though no matter what you do with your stock G1, its limited hardware will not let you multitask very efficiently.

Your other option is rooting your G1 and installing a custom ROM that supports installing applications on the SD card (known as Apps-to-SD or A2SD), and other tweaks that will give you a lot more free internal memory. That is just one of the many pros of a custom ROM, and it's definitely worth it if you find that the stock ROM is lacking.

Sounds like you need to do some routine maintanence on your G1. What you want to do is clear all of your cache out about once a week or more often, depending how often you use it. Clear your browser, market, gmail, google maps and any other browser you have.

If your home screen is full of widgets or apps tone it down a little because the home.app gets out of memory and if your home is full it can take a minute to load but if you tone down the apps it wont be as bad.

Lastly I would recommend that you download the Ultimate fav app so that you can swicth your favorite apps without going home all the time. Hope this helps.