What "compress PNG" can and cannot mean
PNG is a lossless format. That single fact decides everything about what this tool can do for you. Losslessness is a promise that the pixels coming out are bit for bit the pixels that went in, so there is no quality setting to lower and no detail to throw away. All that is left is packing: describing the same pixels in fewer bytes.
That is why a screenshot straight off your desktop might shrink by a third while an asset exported from a design tool barely moves. The design tool already did the packing well. Anyone promising to halve every PNG losslessly is either changing the format without telling you or quietly reducing the colour palette, which is a loss, just an unlabelled one.
The route that actually works
When you need a genuinely smaller file and the image is going onto a web page, a banner, an email or an app, change the format. Set the output to WebP: it stores transparency the same way PNG does, and it is allowed to spend fewer bits on detail nobody looks at. In our own testing the same test image went from 53 KB as a PNG to 15 KB as a WebP, a reduction of seventy one percent, at a size where the difference is invisible on screen.
Keep PNG when the file is going into further editing, when it holds an icon or a logo you will scale later, or when you need a format every ancient piece of software will open. Use WebP or JPG when the file is being viewed rather than edited. Nothing here will change your format without you choosing to.
Why this one tells you when it failed
Most compressors always show a result. If the re-encoded version was larger, they either show that number as though it were a win or silently pass the original off as their own work. This one compares the two and keeps whichever is smaller, then labels it. Being told a file is already optimal is useful information; being handed a bigger file and a green tick is not.
So the useful question is not which png compressor squeezes hardest, but whether the one you are using tells you the truth about what it did. If your goal is to reduce png file size for a web page, the format switch above will do more in one click than any amount of lossless repacking.
Everything happens in your browser. Files are never sent to a server, there is no queue, and a folder of fifty icons is processed one after another so a phone does not run out of memory partway through.