I don't believe device encryption is reversible. I was only able to switch off "require secure password at device startup" - which was even more annoying than the lagginess caused by encryption - my phone would reboot abruptly about once a week when having trouble with a call and switching cell beacons - and then on reboot I would need to enter my long freakin' password. I switched that off, so now I only need the fingerprint (and still have a backup password in case my finger is greasy or sweaty or whatever.
BTW - set up fingers/thumbs on either hand to unlock your phone!