Fast because nothing is uploaded
Online converters usually upload your video, convert it on a server and make you download it again, which takes long for large files and means a stranger's server holds your video. This converter works inside your browser. A one gigabyte phone video never leaves your computer, and there is no queue, no size limit and no watermark.
A video file is a container with a video track and an audio track inside. If those tracks already suit the new container, for example H.264 video in a MOV file going into MP4, they are copied as they are. That takes seconds and keeps the original quality.
Which format should I use?
MP4 with H.264 video and AAC audio plays on every phone, computer, TV and website, so it is the best choice for sharing. WebM is the open web format, compact and well supported in browsers. MOV is Apple's format for editing in iMovie and Final Cut. MKV can hold almost any track and is popular for archiving. When a re-encode is needed, the converter uses the hardware encoder of your device.