Because encryption isn't slow, and neither is decryption. It's actually incredibly fast. The slow part is key generation, but even that isn't noticeable to a regular user.

On a modern CPU you can use AES 256bit at something like 80GB/s.

The decryption process is doing almost nothing to performance, you can decrypt 2GB of encrypted data just as fast as your HDD can read 2GB of data, assuming a somewhat decent CPU, and AES128bit (I think it uses 192-bit, which is something like 15% slower than 128bit).