Hi, Thanks a lot for posting your query here. I am Jins here to assist you. Actually imovie is designed to work with .mov files. It can't identify mp4 files. So, while importing mp4 files it will re-encode the file and that's why you are facing this issue. So, please use a converter program to convert your mpl files to mov format. Then import that mov file to imovie.
I like to recommend you a free video converter named MPEG Stream Clip. Click here to download it.
Install and launch MPEG Stream Clip. Drop your file onto the middle of the MPEG Streamclip window and then select the File menu and then Export to Quicktime. Set the compression type to Apple Intermediate Codec, leave the quality at 50% and sound at uncompressed, stereo, auto. Select the frame size if needed. If the source was interlaced, check the box next to Interlaced Scaling. If the source was originally 720p then you can uncheck this box. That should be it. Click Make Movie and then import the file it makes to iMovie.
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