If you want to use a lot of big images (like in my theme) you should further compress the png's exporting them to indexed mode 256 colors.
Tetsuo is right, but regardless if you use a lot of big PNG-8s or PNGs-24/32s, you should make sure you are encoding them efficiently. I have seen one WiiFlow theme where the images were ~900% larger than they needed to be. Some software will compress PNGs very aggressively (I would say Photoshop 2021 is on average about 98% efficient), but some software, not so much. In order to compress a PNG efficiently, a number of different CPU intensive algorithms have to be tested on that particular image, which I suspect is why not all software bothers to do this.
If you're using a Mac, I highly recommend trying the free ImageOptim (I'm sure there are similar free alternatives on all other major platforms). It's default settings will only losslessly re-compress images, which means it makes them smaller while keeping the quality pixel for pixel identical. Running just a few images through there, will give you a sense if your PNG authoring software is already doing enough, or if it's producing bloated PNGs that would benefit from better compression.
If even the re-compressed the file sizes you are are seeing are still larger than acceptable to you, you can also enable lossy compression, where it will degrade the image quality at a preset amount in order to reduce the file size even more. This is identical in principle to what Tetsuo was recommending with 256 colours, except that it reduces the colour palette a variable amount, based on the amount of image degradation you are willing to tolerate. The reduced amount of colours increases the efficiency with which a PNG can be compressed.
Please feel free to PM me if you wanna know more!
-a
p.s. Link to ImageOptim https://imageoptim.com/ & link to some Linux & Windows alternatives: https://imageoptim.com/versions.html
Last edited by AdamCatalyst,