It seems a good chunk of that is not coming from the processor itself, but from other stuff like how fast is the memory, the storage, etc. So it might not have a better processor than Galaxy S3 or Nexus 7, but it may have slightly better storage and RAM memory.
Anandtech is reporting that the iPhone 5 is using dual-channel 32bit LPDDR2 running at 1066MHz, about 33% more peak memory bandwidth than the iPhone 4s.