CLISP doesn't use GMP; it has its own, very nice, bignum library. If you want to use GMP on SBCL, the SB-GMP contrib has been around for approximately one year; it'll hook transparently in SBCL's bignums, at runtime. However, I think the grandparent was referring to performance on typical numerical computing workloads: machine integer or floating point arithmetic, mixed with a fair amount of array indexing.