Luckily I was trying this through Setapp, so I'm not out any money. I only tested one of Squash's uses; PNG compression, to see if could serve as an acceptable replacement for my current app; Pngyu (free). Since Squash is a paid app (that I was getting to use for free), I was expecting it to be at least comparable.
For my test, I used 12 png files generated from Photoshop that ranged in sizes between 177 KB and 220 KB.
First I tried Squash's standard setting. Compression took 7 seconds and the file sizes dropped to between 159 KB and 202 KB. Not much savings.
I went into the apps settings, and turned on the "More Compressed (Slower)" option, and then ran the original files through again. "Slower" was right. It took 1 minute, 12 seconds to complete, and the range of compression was slightly better at 131 KB to 174 KB... still not what I would call great.
I ran the same original files through Pngyu on the default compression, and the results ranged between 62 KB and 75 KB, and it was completed in 2.8 seconds.
Then I compared them visually and couldn't see any difference between any of the compressed files.
So, if you need an app that compresses PNG files, I would not recommend Squash over Pngyu, even if the prices were the same.
As it is, Pngyu is free, and the speed, options for compression, and default compression setting all blow Squash out of the water.
Since I didn't test any of the JPG compression options, I'm splitting the stars between 0 and 5, at 2.5.