Mounting external drives is SUPER slow

[motrek]

... I suspect that the delay in showing the password prompt is due to doing a file system check prior to decrypting, and the delay after is from doing another file system check on the decrypted file system after decrypting. ...

I'm not sure how it would do a file system check before decrypting. I would expect the file system data structures to be encrypted. So I'm still confused. I would expect MacOS to display the password prompt almost instantaneously, since, what else can it do with an encrypted drive?