I had 234 legacy WMV/RM videos on a NAS: about 42.8GB and 78.5 hours in total. The goal was to create compatible MP4 copies without touching the source directory, while keeping quality, size, power use, and restart safety under control.

The final choice was h264_videotoolbox -q:v 50. On a representative 720p clip it ran roughly twice as fast as x264, produced a similar file size, and reduced the sustained CPU load and fan noise dramatically.