The PIX has a single CPU to process a variety of tasks; for example, it processes packets and prints debug messages to the console. Each process has its own purpose, and some processes require more CPU time than other processes. Encryption is probably the most CPU-intensive process, so if your PIX passes a lot of traffic through encrypted tunnels, you should consider a faster PIX, a VPN Accelerator Card (VAC) [Part # PIX-VPN-ACCEL] for the PIX, or a dedicated VPN Concentrator, such as the VPN 3000. The VAC offloads the encryption and decryption from the PIX CPU and performs it in hardware on the card. This allows the PIX to encrypt and decrypt 100 Mbps of traffic with 3DES (168-bit encryption).

Refer to Monitor and Troubleshoot PIX 500 Performance Issues for more information

http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a008009491c.shtml#showcpu