Some processors (CPUs) support hardware-accelerated AES encryption,* which is typically 4-8 times faster than encryption performed by the purely software implementation on the same processors.
By default, TrueCrypt uses hardware-accelerated AES on computers that have a processor where the Intel AES-NI instructions are available. Specifically, TrueCrypt uses the AES-NI instructions that perform so-called AES rounds (i.e. the main portions of the AES algorithm).** TrueCrypt does not use any of the AES-NI instructions that perform key generation.
Note: By default, TrueCrypt uses hardware-accelerated AES also when an encrypted Windows system is booting or resuming from hibernation (provided that the processor supports the Intel AES-NI instructions).
To find out whether TrueCrypt can use hardware-accelerated AES on your computer, select Settings > Performance and check the field labeled 'Processor (CPU) in this computer supports hardware acceleration for AES'.
To find out whether a processor you want to purchase supports the Intel AES-NI instructions (also called "AES New Instructions"), which TrueCrypt uses for hardware-accelerated AES, please check the documentation for the processor or contact the vendor/manufacturer. Alternatively, peruse this official list of Intel processors that support the AES-NI instructions or use the official AMD website to find such AMD processors. However, note that some Intel processors, which the Intel website lists as AES-NI-supporting, actually support the AES-NI instructions only with a Processor Configuration update (for example, i7-2630/2635QM, i7-2670/2675QM, i5-2430/2435M, i5-2410/2415M). In such cases, you should contact the manufacturer of the motherboard/computer for a BIOS update that includes the latest Processor Configuration update for the processor.
If you want to disable hardware acceleration of AES (e.g. because you want TrueCrypt to use only a fully open-source implementation of AES), you can do so by selecting Settings > Performance and disabling the option 'Accelerate AES encryption/decryption by using the AES instructions of the processor'. Note that when this setting is changed, the operating system needs to be restarted to ensure that all TrueCrypt components internally perform the requested change of mode. Also note that when you create a TrueCrypt Rescue Disk, the state of this option is written to the Rescue Disk and used whenever you boot from it (affecting the pre-boot and initial boot phase). To create a new TrueCrypt Rescue Disk, select System > Create Rescue Disk.
Note: Support for hardware acceleration was introduced in TrueCrypt 7.0.
See also: Parallelization