No, the subtitles are already in SRT. No video decoding is happening. ffmpeg was using 2% CPU and 0% GPU. The bottleneck is IO. ffmpeg must read the entire file

No, the subtitles are already in SRT. No video decoding is happening. ffmpeg was using 2% CPU and 0% GPU. The bottleneck is IO. ffmpeg must read the entire file, parse the container format and collect the subtitles. If the SSD was completely idle and could reach it's max rated read speed of 550 MB/s, assuming absolutely no overhead, ffmpeg would finish in just over 2 minutes (70000 MB / 550 MB/s / 60 s/min = 2.12 min). So, in the best case scenario, I'll be stuck waiting for 2 minutes for the subtitles to appear, if the client doesn't time out. Real-world speeds are 10x slower, as we've seen, but they're more than 10x faster than 4K Blu-ray bitrates, so they're sufficient. How large is your 47 minute video? If it's ~8 GB, which is typically on the high end for episodes from streaming services, and if you have an PCIe 3.0 NVMe SSD, which reads at ~6 GB/s, you'll extract subtitles in about 1 second.