Lisp is actually really light weight in most things, though having to support continuations (if you're using a dialect that has them) adds some overhead. It's not quite Forth-ish in its lightness, but it's not that far off.

Part of Lisp's sluggishness comes from its very nature--it's really awkward to do a lot of things in Lisp. It just doesn't lend itself well to many problems. (For those you apparently need to use Scheme, which rumor has it will end world hunger, bring about world peace, and aid in the development of a really good tasting diet soda...)