I noticed your Portfolio page loads the details for each site in the background. Great from a perceived speed standpoint because each detail loads instantly when chosen. However…your thumbnails are all retina-sized PNGs. The total size for the 9 images on this page was 10.4MB! That’s very heavy and kills the speed benefits gained from preloading, especially when the resulting images are so small. On mobile only the first thumbnails are shown, so this already huge size eats up tons of bandwidth with nothing to show.
Shrink the thumbnails to the smallest size you can get away with and consider linking to the larger images if you want to show detail. Small changes, huge gains!