The front page loads with 10 HTTP requests on an empty cache, and doesn't have any user-perceptible blockage while waiting on elements. (Pulled this straight out of YSlow.)
Improvement is overkill, but if you wanted to:
1) Creates images{1,2}.ycombinator.com and split y18.gif, s.gif, and grayarrow.gif, and your JS and CSS files across the three domains. This will cause older browsers to load them all in parallel. (The HTTP spec suggests having no more than 2 requests to any one domain open at a time, so you end up with stairstep loading graphs on browsers which are spec-compliant, such as IE6. Browsers which have a more pragmatic view towards compliance with published specifications, such as Firefox 3, will do 8 ~ 10 requests in parallel.)
Note the balancing act in that every domain you add is another DNS query that needs to get resolved once. Ideally you want to keep it to about 4 domains or less.
2) Put far future expires headers on your static assets. You can bust caches by using the Rails-y filename.js?timestampOfLastModificationOfCode method.
But, again, this site is probably the fastest one I use on a regular basis, even from braindead clients like my Kindle. I wouldn't guess the marginal improvement is worth the expensive engineer time.