Question about Truecrypt encryption on an HDD

Hi,

I'm am uncertain about how or if encryption on a boot drive would affect performance after the encryption code has been entered in?

For example, I have a 1TB HDD that has Windows 7 Home installed on it. On this drive, it requires a truecrypt password to unlock the drive in order for it to start booting. After I enter the password, it boots and all, but my concern now is if it needs to further decrypt other files now that I unlocked the drive for full use?

It just needs one password and the whole drive is unlocked. Does it need to further decrypt other files after the drive has been unlocked?

I'm using a 3-cipher encryption method for the drive just to boot into it. All the files are unlocked after booting in.

Since Truecrypt also has an option to mount encrypted drives while using a different HDD, would I get the same performance as a non-encrypted drive after it's unlocked? But the only difference is being this drive is a storage center for games, documents, etc, the works.