I think a useful test for this case could be pretty simple. Compress maybe 10% of the file (up to some reasonable maximum, like maybe a few megabytes) and check

I think a useful test for this case could be pretty simple. Compress maybe 10% of the file (up to some reasonable maximum, like maybe a few megabytes) and check the compression ratio. If it's not something better than maybe 1.1x, give up, throw out the compressed data, and just pass through the file. Otherwise keep going.