Cryptographic operations are complex and can introduce performance problems and also on mobile devices, battery life is another resource need to be critically managed. Encryption of data is deeply built into iOS.
iOS security documentation (page 9) goes in some detail on explaining how the encryption mechanism works.
Is it all Symmetric Encryption?
Yes, iOS uses AES algorithm with 256 bit keysize.
How is performance taken into account with this heavy load of encryption?
Quoting from the document.
Every iOS device has a dedicated AES 256 crypto engine built into the DMA path
between the flash storage and main system memory, making file encryption highly
Thus, by using a hardware encryption engine, and integrated at the very basic IO level helps in getting the performance and also in return saving battery life.
EDIT: In Blackhat Europe'15, an excellent talk was given on Disk Encryption on mobile devices (iOS and Android). The whitepaper for it covers encryption in iOS in very simple and straightforward manner.