Veracrypt and it's predecessor Truecrypt transform plaintext disk content to cyphertext disk content on the fly.
There is a one-off process to convert the plaintext to cyphertext when commissioning the hard disk.
So this is a full disk re-write which is limited primarily by the speed of the hard disk. A spinning hard disk is the slowest part of the typical memory hierarchy (CPU Cache, Main Memory, SSD Cache, Hard Disk) and there are very few ways to optimise this process (i.e. the cache will not speed it up).
The on-the-fly rewriting of content is practically unnoticeable in comparison, especially on modern CPU's.