We, Body Boss ( http://bodybossfitness.com ) didn't even have a product last time year, and we thought just having a good idea was enough for acceptance. In the past year, we've boostrapped like crazy, built a product, launched and acquired plenty of customers to validate our idea. YC rejection for the 2nd year in a row has done little to dampen our spirits and ambition. Here's to another rejection that motivates us…
1 - Some of your images are huge. The ipad workouts image alone is 300kb, which is way too big. Of course you want them to be good quality, but you should be able to cut a lot of weight without any noticeable effect. The Fireworks Export Wizard is a good tool for this.
2 - Some of your images are unnecessarily broken up into lots of separate img tags. The main landing page image with the helmets and devices has over 6 tags. You should combine images in order to have as few img tags as possible. It will reduce http requests and also give the browser less layout work to do.
3 - On a similar note, you have a bunch of small images like the player portraits that you could group into a spritesheet to further reduce requests.
4 - It doesn't seem like you're using a CDN? Something like Cloudfront or Cloudflare is quick to setup and will improve download time for your images drastically, especially for people who are far away from your server like me.
5 - You should put your javascript at the bottom of the body instead of in the head. You're not executing anything until the dom is loaded anyway, and putting at the bottom will allow the page to render before the js is downloaded and parsed, improving perceived load time. Use 'display: none' to initially hide anything that depends on the js to look right.