ffmpeg – How do tools like CapCut and Veed.io add subtitles so quickly?

I’ve tested video editors like CapCut and Veedio, and I’m amazed at how fast they can add subtitles to a video. I’m curious about how they achieve such impressive performance.

I’m considering two possibilities:

• They render a single canvas that combines the video and the subtitles, so the output is the same as what you see in the editor.

• They create a separate subtitle track and attach it to the video later using something like FFmpeg.

Do you think they use a different method to achieve such fast video rendering with subtitles?