resize and compress and maybe even dither your images! you can see the changes while you're tweaking settings! supports a bunch of encoders! (you probably just want OxiPNG and MozJPEG though.1)
i use it all the time, it's super handy.
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and maybe WebP. AVIF can be nice, but it's really slow, and the other options are either worse or not natively supported in browsers.