Essential Image Optimization
I had a landing page with 37 high quality images spread across 5 pages. Used Squash at first to bring down the aggregated size of the images from ~550MB to ~320MB and voila my bounce rates started going down. A few months later i tried out Cloudinary and the conversion rates improved since the biggest bottleneck of the landing page was the images and they were loading smoothly. IMHO one of the low hanging fruits that is worth solving.