Depends on the codec change. Same-codec re-mux: 30-60 seconds (no re-encode). Re-encode to a different codec: typically 0.3-0.7x source duration on our GPU pipeline, so a 1-hour WebM -> FLV finishes in 18-40 minutes; AV1 re-encodes are 2-3x slower.
Not in the same step — use /video-trim/ or /video-cutter/ to clip the WebM first, then queue the WebM -> FLV conversion. Trim-then-convert is faster than re-encoding the whole file just to crop the output.
Yes — embedded subtitle tracks (mov_text in MP4, SRT / ASS in MKV, WebVTT in WebM) are preserved when both WebM and FLV containers support them. Burned-in (hardsub) subtitles transfer automatically because they are part of the pixel data.