What does 'lossless remux' mean?

When the source's video and audio codecs are already legal in the target container (H.264 in MOV → MP4, for example), the streams are just repackaged rather than decoded and re-encoded. It's instant and the quality is identical.

Which formats can I convert?

MOV, MKV, AVI, WebM, MP4 and even GIF as input; MP4, WebM and MKV as output.

Why does WebM always take longer?

There's no lossless path into WebM from typical sources, so it re-encodes to VP8 + Opus. That's real work, and WASM makes it slower than a native app would be.

MP4 to MKV Converter Free — In Your Browser, No Upload

MKV holds essentially anything, so an MP4 nearly always remuxes into it losslessly — same streams, new container, no quality change and no waiting for an encode.