I believe that requires a custom ROM (or at least a custom recovery?). I doubt it's worth the time it takes to do it (and the time it will take to return to stock for the next OS update). Note that improvements in 5.1 and 6.0 had sped up software encryption/decryption quite a bit over the initial tests run on memory I/O by places like Anandtech. The biggest issue now is that it takes extra CPU cycles, so there's a small (but not necessarily trivial) amount of battery life to be saved with hardware encryption/turning it off. Assuming you aren't running out of battery every day then the only time you'll see a significant difference in use with it decrypted is during app installation, or the opening and closing of larger apps (e.g. games).
That said, if you still want to do it XDA has several posts on it.