Are you having to render the entire sequence every time you add a clip to it? If so, your sequence settings are probably wrong for the type of footage you're using. Do your sequence settings match your clips settings? When you added the first clip, did it ask you if you wanted to change the sequence settings to match the clip settings? What exactly are you doing? What kind of footage are you using and what are your sequence settings?
If you're adding, say, DV NTSC footage to a ProRes422 sequence, everything you add will have to be rendered. Your sequence settings would need to be changed to DV NTSC.