Enabling FileVault on an older Mac, particularly one with a hard drive, will take a long time because it has to encrypt everything, rather than just changing the key to one you control. Thus, if ...

You can boot your laptop, desktop, or Mac using the bootable USB and select the hard drive or SSD you want to erase. The program will then overwrite every sector on the drive with random data, ...

Unlike the hard to use internal Secure Erase function, these methods of overwriting or encrypting are easily accessed in Windows and Mac OS. Protect your data and sleep in peace.