Right-click on the clip (or Control-click) and see if it offers to optimize. If so, say Yes.
You can also right-click on the clip in the Event and see if it offers to optimize. If so, say Yes.
If it does not, come back and I will give you a different way to optimize.
If you want to understand what is going on when you optimize, your camera shoots in a very compressed format. This is usually either AVCHD, h.264, or MPEG2. These are called Long GOP (group of pictures) compression techniques. See my post here for why it is good to convert to i-frames (or optimize) before editing.
Can you right-click on this clip and select Show In Finder (or Reveal In Finder). In the Finder, right-click on this clip and select OPEN WITH.../QUICKTIME PLAYER. In QuickTime Player, type the letter i while holding down the Command Key. This will open the Inspector. Tell us the clip dimensions and tell us everything you see next to the word FORMAT.