Well,
A while ago.. 2 years ago .. I had too problem of slow cache ie. squid and lots of tcp_miss .. So that time I tried a lots of things.. including changing filesystem reiserfs , changing file descriptor limits, changing squid thread limts and few other things..
You may try this who is addressing similar thing.. ofcourse you dont have to follow exactly but try to do tweaking this way too.. (I must say that its a nice tutorial)
http://www.linux-faqs.com/squid.php