Why WebM to MP4 is the slow conversion
Elsewhere on this site, container conversions are mostly instant, because the video inside can be copied into the new file untouched. WebM to MP4 is the exception, and it is worth understanding why: WebM carries VP8 or VP9 video, and the world that cannot play WebM, chiefly iPhones, Safari, QuickTime and desktop editors, cannot play those codecs inside an MP4 either. Changing the box would change nothing.
So the tool does the real thing. Every frame is decoded and re-encoded to H.264, the audio becomes AAC, and the result plays on effectively every device made in the last fifteen years. The cost is time: in a browser this runs slower than the clip's own duration, with a progress bar and a working cancel button. We would rather tell you that up front than let a spinner imply otherwise.
When this tool is the right choice
Short material is the sweet spot. A screen recording to drop into slides, a browser-recorded clip an editor refuses, a downloaded snippet that needs to play on a phone: half a minute of video converts in about a minute, entirely locally, with nothing leaving your machine. That last property matters for screen recordings especially, which tend to contain things you would not paste into a random website.
For an hour-long recording, be honest with yourself about the arithmetic and use desktop software. The browser will get there, but it is the wrong instrument for that scale.
What comes out
An .mp4 holding H.264 video and AAC audio at a bitrate chosen from the source's resolution and frame rate, with the index at the front so playback starts immediately. Files up to 500 MB are accepted. The first run fetches the conversion engine once, and the page states its size before anything starts.