Adjusting Speed of Multiple Clips iMovie

As I understand your post you are working with still clips and you want to change the duration of some of them to 4.5 seconds. The term "speed" refers to frames per second and applies only to video clips. When dealing with still clips, the term is "duration", not speed, because there are no frames per second to speed up or slow down. The Edit/Paste Adjustments function only applies to speed, not duration, and that's why it is grayed out.

For still clips that are contiguous to each other, you can select them as a batch (range), adjust the duration to 4.5 seconds using the clip info button, and then press on the return key on your keyboard. That will adjust all of the clips in the range to 4.5 seconds. If there are stills in the range that you don't want to change to 4.5 seconds, you will need to go back and readjust their durations to what they were before. Or, you can select ranges that exclude the clips whose duration you don't want to change.

-- Rich