Hi Nick,
I am having the same issue. We have a 2851 as a IPSEC VPN headend with an AIM VPN module but we are seeing high CPU usage(80%) with just 4-5mbps worth of traffic. I have an idea that I might have a NAT issue.
We are currently running, NAT, ZFW, and IPSEC site 2 site VPN on the router.
When I look at my ZONE firewall policy-map output it is showing all of my VPN traffic as process switched.
Inspect
Packet inspection statistics [process switch:fast switch]
tcp packets: [14809800:0]
udp packets: [145107:0]
icmp packets: [20937:12]
I have disabled the ZFW and still see high cpu although it is a little lower.
Packets are not fragmented, CEF and fast switching looks to be enabled. I am using a route-map for my nonats. That is the only thing I can think of now.
I have tried IOS 12.4(20)T3,4 and 12.4(15)T9. Same results.
Anyone have some ideas?