It's interesting that Mozilla has chosen to use an image with so much red, which makes JPEG (at their settings) look relatively poor even at quality 100. You cannot optimise the limitations of 2x2 chroma subsampling.
I am also not convinced by Trellis yet. In my tests, it introduced a faint blur. In fairness though, I only tried 4:4:4 and not 4:2:0 as Mozilla apparently does. In the latter case, additional blur may not be noticeable.
You can 'optimize' encode-time by using better chroma downsampling, but really the issue is usually on the client side upsampling (a lot of libs use terrible nearest neighbor for 'speed').
As for trellis, you should play with the different metrics available (MS-SSIM etc), but I agree I have not been wowed yet.
Yes, optimise is the wrong word. I meant optimise away really, but that's still not quite right. The WebP authors are actually working on improving chroma subsampling, because VP8 limits WebP to 2x2.
I am also not convinced by Trellis yet. In my tests, it introduced a faint blur. In fairness though, I only tried 4:4:4 and not 4:2:0 as Mozilla apparently does. In the latter case, additional blur may not be noticeable.