I go back and forth on this; if I use ffmpeg and leave fast start on it for some reason, it compiles the file on my external drive H: HD and takes a long time.
I was a long-time user using mkv; overall it might be better to use mkv.
If I do use ffmpeg, I leave fast start on, and my separate SSD drive does the brunt work, and it’s still at 100% as it is a WD higher-quality SSD and 4 TB in size as well. My C: drive, a 1 TB NVMe, is still hanging in at 95%.