There are no official benchmark docs but I do the testing / vetting for each NAS on the list.
I can tell you . based on passmark scores of the J3455 series processors, you can get about 3, guaranteed 2, audio conversions with DirectStream from those CPUs.
The performance scaling is pretty true - linear.
The single biggest concern with any CPU will be subtitle conversion / burning (if you need it). Subtitles are single-threaded so , with high bit rate video, you need a CPU which can burn the text into each frame at that resolution/bit rate very quickly.
Burning subtitles into 80Kbps video with an i7-7700 will chew it up pretty quick.
2-3 playbacks and the CPU is full.
