What speed of ffmpeg should one expect from a-shell?

Hello, apologies if this isn't a proper type of question - I'm quite new to this type of program on iOS/iPadOS.

My goal is to find the best performing ffmpeg implementation that would work on files stored in the regular files.app on an iPad.

My understanding was that iSH should in theory be slower than A-Shell, due to implementation, however I found that to be the opposite. Sample command was to merge a .srt file into a .mkv file, both files residing in a local folder in files.app. Code used in both apps:

`ffmpeg -i input.mkv -i input.srt -map 0 -map 1 -c copy output.mkv`

You'll note this isn't re-encoding anything. Done on a 30GB mkv file (3h movie file), I get the following:

iSH: approx task speed of 6x (task completes in 30 minutes)
A-Shell: 2.4x (task completes in 1h 15 minutes)
Intel Macbook Pro with homebrew ffmpeg, for reference: 100x and above (task completes in less than 2 minutes)

Anything I am missing here?