Hi Tom,
If you are in the same Mac camp as I, Apple’s Final Cut Pro X with Motion 5 will be my preferred vehicle for videos. But having said that, you can get away with a lighter, friendlier tool like iMovie for all your Nikon video clips.
Adobe Premiere Pro rocks on Windows too! (Well, at least from the smooth work I saw off my videographer peers). As for lags, I implore you to confirm the fit-for-function of your PC’s graphic card (and RAM Size) before properly working on your HD clips.
Most of my video works are performed on 8GB machines…