In the Timeline of a project, can iMovie do highspeed playback?

Yes, you can view the project at high speed without editing. I had forgotten about the feature.

With the keys J, K and L you can toggle forward and backward through the time line. If you tap them multiple times you will go faster with each tap, -- J to go backward and L to go forward. Tap K to stop. So, if you want to speed up going forward just keep tapping the L key to go faster and faster.

You an also drag your cursor along the timeline to speed up.

-- Rich

You can do Edit/Select All, and then Modify/Fast Forward. Do Modify/Reset Speed to get back to normal playback.

You also can do Edit/Select All and use the speed adjustment feature in the tool bar at the upper right of your screen, to fast forward.

-- Rich

If you want to turn off the audio using the L,K,J method, do an Edit/Select All and mute the audio using the loudspeaker in the toolbar at the upper right of your screen.

If you are using the drag the cursor method you can turn off the audio by clicking on View and unchecking Audio Skimming.

-- Rich