Videos are not converted after upload. Viewers receive exactly the file you upload, so please prepare it as follows:

• Format: MP4 with H.264 video and AAC audio. Don't use H.265/HEVC, because many browsers can't play it.
• Web optimised (fast start): this lets playback begin straight away. In HandBrake, tick Web Optimized. In ffmpeg, add -movflags +faststart.
• Resolution: 1920×1080 (1080p) is preferred. Use 1080×1920 for vertical videos. Don't upscale smaller videos, and don't upload 4K.
• Bitrate: about 5–8 Mbps for 1080p keeps playback smooth on phones and slower connections. As a rough guide, that's about 40–60 MB per minute of video.

Tip: HandBrake's Fast 1080p30 preset with Web Optimized ticked meets all of the above.