How can FileVault encryption process be stopped/disabled?
Unfortunately it is not possible to stop FileVault encryption midway.

Your only options are to let it continue, or to erase/format your drive and restore from backup.

The encryption process takes place in the background allowing you to use your computer while encrypting. Usually it should not make your computer extremely slow, so it might be worth investigating if you have something else running that is making your computer extremely slow. Take a look at Activity Monitor to see what the cause might be.