What re-encoding actually costs you
Most video editors decode your clip and encode a brand-new one. Every lossy re-encode discards detail that never comes back, and it takes real time — minutes for a long 4K clip.
A trim does not need any of that. Cutting the ends off a video is a copy operation: keep the stretch of frames you want, drop the rest, and leave the compressed data untouched. This is usually called stream copying or passthrough, and it finishes in seconds at the original quality.
Option 1 — QuickTime Player
Open the clip, choose Edit → Trim (⌘T), drag the yellow handles to the section you want, click Trim, and save. For formats QuickTime handles comfortably this keeps the original streams. It is free, already installed, and for a straightforward MOV it is often all you need.
The limits appear with less common containers and codecs, when you want a cut at a specific frame, or when Export is the only option that will accept your file — Export re-encodes.
Option 2 — a dedicated trimmer
SnappyEdit trims by copying the original video and audio streams into a new MOV, so the output holds the same data your camera recorded. You set a start and an end, export, and the source file is never touched. It reads the containers AVFoundation supports, including MOV, MP4, M4V, AVI, 3GP, MPEG, AVCHD, MTS/M2TS, DV, and VOB.
When you do want a re-encode
Lossless trimming keeps the original codec, which is not always what you need. Re-encode when the destination device or site will not play the original codec, when the file has to be smaller, or when the resolution has to change. SnappyEdit exports H.264 up to 4K, 1080p, and 720p, and HEVC up to 4K and 1080p, in the same operation as the trim.
How to tell whether your trim was lossless
Compare the output size against the running time. A lossless trim of half a clip lands near half the original size, while a re-encode usually lands somewhere unrelated. Timing is the other tell: a passthrough trim finishes almost instantly regardless of clip length, because nothing is being compressed.