Interesting points.... but you have a basic misunderstanding about what File Vault does, and how it works..
Did you notice that you can continue working normally (or doing whatever it is you do with your Mac) while the encryption process continues?
There is, in most situations, very little noticeable performance decrease while either encryption, or decryption takes place. (That would be part of its "poor engineering"
Notice also that when you log in to an encrypted drive, that you would seldom even know that the drive is encrypted, other than the slight change in opening the drive with a password.
Finally, turning on (enabling) File Vault encrypts your hard drive.
Once the encryption is complete, you would leave File Vault ON. File Vault automatically encrypts new information. There's nothing that you need to do, other than remember the login password, and also the File Vault key that is generated when you first turn on File Vault.
Turning OFF File Vault removes all encryption (and you are back to an un-encrypted volume, back to square one, eh?)
So, if you want the extra security of File Vault (and it is quite secure), then enable File Vault. The initial process takes some time, and you can continue to work normally while that process occurs. You leave File Vault turned on, it takes however long it takes. New information, new data, your added files, all are encrypted when you copy those to a File Vault-enabled drive. That additional process takes very little time, and you won't notice any difference as the process is essentially transparent.
The important point is that you leave File Vault turned on, and that takes care of itself.
You would only turn File Vault off if you want to remove the encryption from the drive completely. Yes, that also takes time. But, it is a process that you probably won't ever need to do, unless you need to turn File Vault off because of some problem that you have with the drive.
You DON'T ever need to turn File Vault off just so you can use the drive. Your data is always protected while File Vault is on, and you can use your drive as normal, even while encrypting, and AFTER the encryption is complete.
BTW - you don't need to turn off File Vault (or de-crypt) your boot volume to reinstall macOS. You just enter your unlocking password when you choose an encrypted volume as the install destination.