> Maybe a lossless re-encoding of my website JPEGs to gain 20% size is not worth the 33% longer decode,

And pray tell how much of the waiting for page is used on decoding image (vs transferring) and how much faster the image gets on the machine when it's 20% smaller on average connection?

My guess is that time saving from bandwidth decrease more than compensates for that

Well it obviously depends on the connection, but at 3g speeds and higher, the decoding time outweighs the fairly minor bandwidth saving.

And if you're displaying a cached image, the bandwidth use is zero, but the decoding still happens.