This is extraction, not conversion
An MP4 is a container. Inside it sit separate streams: usually one H.264 video track and one AAC audio track, interleaved so a player can read both while streaming. So to convert MP4 to MP3 is really to extract audio from MP4 and re-encode it: find the audio stream, decode those samples, hand them to the MP3 encoder, and discard the video entirely.
That distinction matters because it explains the two things people find surprising. The first is speed: no picture is ever decoded, so the job is bounded by how long the audio is, not by whether the video was 4K. A ten minute 4K clip and a ten minute 480p clip take almost exactly the same time. The second is size. A 400 MB video routinely becomes a 9 MB MP3, and nothing is wrong: the video was almost all of those bytes.
The honest cost
The audio in an MP4 has already been compressed once. Re-encoding it to MP3 throws away a little more, because the MP3 encoder has no way of knowing which detail was already discarded and simply makes its own decisions again. This is called generation loss, and it is the reason a chain of conversions degrades while a single one usually does not.
In practice it is a small price for a format that plays on absolutely everything. Pick 192 kbps and most people will not hear a difference on the gear they actually use. Pick 320 kbps if the file is going somewhere you cannot re-do, and 128 kbps if it is speech, where the extra bitrate buys nothing.
What the tool will not do is pretend. It keeps the source sample rate rather than resampling to 48 kHz, and it never claims to improve anything, because decoding and re-encoding cannot recover detail that is already gone.
Size, speed and the ceiling
Files up to 500 MB are accepted. Above that a browser tab genuinely cannot hold the container in memory, and being told so up front is better than having the tab killed halfway through. If your video is larger, trim the part you need first.
Everything runs locally. The engine is ffmpeg compiled to WebAssembly, fetched once and then cached by your browser, so the second file you convert starts immediately.