With Vegas 9.0a, when I drag an image to the timeline there is approximately a 20 second delay (hourglass) before the program will respond. This makes 9.0A unusable.
If you get lag when coming back to Vegas try changing the settings in your Options>Preferences>General>Close media files when not the active application. If it's checked Vegas tries to basically free up those files and memory if its in the background or minimized and has to reload everything when you come back to it. If it's unchecked (depending on your system) it should come back without the delay.