By default yes — 48 kHz MP3 stays 48 kHz in WAV. If you need to downsample for compatibility (96 kHz → 44.1 kHz for CD burning), the sample-rate option applies high-quality resampling in the same encode pass.
Same-codec re-mux: 10 to 30 seconds. Re-encode to a different codec: typically 10 to 20% of source duration, so a 1-hour MP3 → WAV finishes in 6 to 12 minutes. Batch jobs parallelize across workers for further speedups.
Why is the WAV louder or quieter than the MP3 source?