James Gosling (Java creator) on the object/primitive gap: 
>From http://fishbowl.pastiche.org/archives/001452.html

Depends on your performance goals. Uniform type systems are easy if your
performance goals aren't real strict. In the java case, I wanted to be able
to compile "a=b+c" into one instruction on almost all architectures with a
reasonable compiler. The closest thing I've seen to accomplishing this is
"Self" which gets close, except that the compiler is very complex and
expensive, and it doesn't get nearly all the cases. I haven't read any of
the squeak papers, so I can't comment on it.

Found on James Robertsons Weblog
http://www.cincomsmalltalk.com/blog/blogView#3238140088