[AlanHK]

I optimise for filesize, which should also decode faster.

You can greatly reduce image file size for linework by reducing the number of colours,
To 2 colours if it's lines or rectangles, to 4 if there are diagonals or curves, as greys make less jagged lines than 2. Irfanview on Windows is very good to do this, can even do a batch on a folder of images.

Also I look at the image size. Sometime it's thousands of pixels, when reducing it to a few hundred shows little degradation on a ebook player.

Use PNG for this, or if you are going to use on Kindle, GIF, as PNGs are converted to JPEG or GIF on conversion with lower quality and larger size.

Sometimes can reduce the image size from JPEG by 95%, and get a better result.