The “Smallest/Slow” compression mode does NOT reduce the quality of the image. The PNG file format features lossless compression (smaller file size but the same quality). Its only disadvantage is that compressing the PNG requires a lot more computation, so the export process takes more time (hence “slow”).
Here is the difference between a normal (progressive) image and an interlaced one:
- When a progressive image is loaded (such as in a web browser), the top part gets downloaded first and the bottom last, so the image progressively appears from top to bottom.
- When an interlaced image is loaded, you first see a blurry version of it, and then the details come in as the image gets fully downloaded. PNG interlacing achieves this using the Adam7 algorithm. Because you can see an overview of the whole image before all the details are actually loaded in the interlaced image, it can be better for the web as users can determine whether the image is worth waiting for to fully load.
What to use
Unless you have an impossibly slow computer and not 10 seconds to spend waiting, I would always recommend using the “Smallest/Slow” mode to generate smaller files. And unless you are exporting graphics for a webpage design, I would also recommend using no interlace, also to get smaller files. Sometimes, a few megabytes make the difference between ease and difficulty of file transfer – take the filesize limits of email attachments as an example.