FCPX 11 120fps transcoded clips doesnt show slow motion controls

iOS/iPadOS 18 introduced QuickTime FullFrameRatePlaybackIntent tag:

"key that represents whether this movie should play at full frame rate

Some apps play movies recorded at frame rates of 120fps or higher in slow motion. If your app records high-frame-rate movies, you can add this movie-level metadata to indicate whether the movie intends to play at the full frame rate (1) or at a slow motion rate (0). Apps that play movies may use this metadata, when present, to guide their behavior."

quickTimeMetadataKeyFullFrameRatePlaybackIntent | Apple Developer Documentation

macOS 15 Sequoia QuickTime Player plays middle part of high frame rate movies (seems to be ≈85 fps or more) in slow-motion without that tag (earlier macOS like Ventura seem to behave the same).

For example, losslessly convert an existing 25 fps .mov video to 250 fps (25/250=0.1) and delete audio:

ffmpeg -itsscale 0.1 -i input.mov -c copy -an output.mov

The whole movie plays at 250 fps in Sequoia after adding or editing:

exiftool -m -P -overwrite_original -api LargeFileSupport=1 -ext mov -Keys:FullFrameRatePlaybackIntent=1 output.mov

Back to the default slow-motion behavior:

exiftool -m -P -overwrite_original -api LargeFileSupport=1 -ext mov -Keys:FullFrameRatePlaybackIntent=0 output.mov

Or just delete FullFrameRatePlaybackIntent tag:

exiftool -m -P -overwrite_original -api LargeFileSupport=1 -ext mov -Keys:FullFrameRatePlaybackIntent= output.mov

BTW it seems currently Windows might be incompatible. If that tag is present, Windows might fail to display thumbnails and most metadata items in its Details panel (Media created, resolution, frame rate, audio details etc).

If that tag causes problems in Windows, you might remove it from all .mov movies in a folder with (the dot in the and means the folder you are currently in -- alternatively you can replace it with the path to that folder):

exiftool -m -P -overwrite_original -api LargeFileSupport=1 -ext mov -Keys:FullFrameRatePlaybackIntent= .

https://exiftool.org/forum/index.php?topic=16824.0

The original clips from iPhone are the ones where you have these slow motion controls. I think that you are mistaking those from clips exported out of FCP.

10.8.1 cannot even create projects at 120fps - it tops out at 60fps.

Also: a 120fps is not a slomo movie, it is a high frame rate movie. The point is to record enough frames so that you can do slow motion, in a standard frame rate like 30fps, with best quality. For example, a 120fps clip played at 25% speed in a 30fps will use all the recorded frames.