How can I trim a very large video file?
ffmpeg

Use ffmpeg to trim the video. It will take a while but should get the job done:

See Alexander Refsum Jensenius's Trim video file using FFMPEG example below for extracting 1h 10s 27 to 2h 18s 51:

ffmpeg -i input.mp4 -ss 01:10:27 -to 02:18:51 -c:v copy -c:a copy output.mp4


See also How to trim a large video with ffmpeg? where the question involves a 75GB movie file. In this case the command extracts 7 seconds from a specific starting time:

ffmpeg -ss 01:21:24 -i input.mp4 -t 7 -c copy output.mp4


Movie Splitter

You could also try my Movie Splitter but I expect if QuickTime is struggling, so will the shared AVFoundation code Movie Splitter uses. Still, give it a try as it does work with multi-gigabyte MP4/h.264 files.