High CPU on Cisco2500 with Encrypt 512k.

My Cisco 2500 router(IOS 11.2) suffers high CPU utilisation (Encrytion process can shoot up to 99%)when using a 512K leased line.How do I configure the router so that it only has a bandwith of 128K which is the max the 2500 router can take when running Crypto?This means to say that my 384K is not use until it is replaced with a more powerful router.