It is not necessary to turn off Filevault to clone the drive if you use Carbon Copy Cloner (don't use Disk Utility to clone a boot drive). You need to have the laptop plugged into a working charger in order for Filevault to decrypt the drive. This can possibly take days if you have a slow hard drive and maybe even forever if the hard drive is failing. Also if you have the 13" model the hard drive SATA Cable may be bad. You can connect the drive externally and boot from it which would bypass a bad cable.
Personally I'm guessing your old hard drive is failing. I suggest checking the health of the hard drive by running DriveDx and posting the report here using the "Additional Text" icon which looks like a piece of paper.
I hope you have a backup since it is unlikely you will be able to recover anything from a failing drive that has been stuck decrypting.