One of the biggest impacts of moving to 64-bit is increased memory pressure. While all of Apple's apps and daemons are running 64-bit, most users will be active

One of the biggest impacts of moving to 64-bit is increased memory pressure. While all of Apple's apps and daemons are running 64-bit, most users will be actively using third party apps that are 32-bit-only for a while. This means that on average there is less memory available in the system, because the amount of RAM is unchanged in the 5s, and there will now be code from both 64-bit and 32-bit binaries resident, rather than just 32-bit binaries. Apple has done some work to alleviate this extra memory pressure at the kernel level. grep for WKdm in the xnu sources if you're interested.