I’ve been tuning Photoshop performance on some decently specced Mac Pro workstations. They’re older machines than the current model and by default don’t boot in 64-bit mode.
Whilst Photoshop runs in 64-bit mode under a 32-bit kernel, you will get better performance with 64-bit apps on a 64-bit kernel.

You can force your Mac to boot in 64-bit mode by holding down a key combo at startup, however this isn’t really practical as a permanent solution.

The easiest way to set the machine to boot always in 64-bit mode is using the systemsetup command:

sudo systemsetup -setkernelbootarchitecture x86_64