How to edit a video held in photos . e.g. to skip frames to play at double speed.

Create a new iMovie project and import your Photos app video into it by clicking on the photos item in the iMovie media sidebar. Then from the timeline, click on the speed button (the one that looks like a clock) in the toolbar at the upper right of your screen. You can adjust the speed of the project or a selected clip from the controls that appear. If the clips contains audio, you can preserved the pitch by clicking on the Preserve Pitch item above the viewer screen.

An even easier way is to Control-click on a clip in the timeline to reveal a pop up menu. From the menu, click on Show Speed Editor. That will cause a speed adjustment handle to appear in the upper right hand corner of the clip. Slide the handle to the left to shorten the clip and increase its speed. Slide the handle to the right to lengthen the clip and slow down the speed.

Here's a link to an online help menu that explains how to adjust speed:

https://help.apple.com/imovie/mac/10.1/#/mov6c442b2eb

-- Rich

Additionally, you can add a video clip to an iMovie project from the Photos app by opening iMovie and Photos, and dragging the clip directly from the Photos app library to an iMovie project timeline. Or you can drag from the Photos app to your desktop and from there drag into iMovie.

-- Rich