Hello,
I have a 1100AH in the data center and a 450G in my home/office. I run 2 VPN so far between the 450G and the 1100AH. The 450G is quite often quite taxes when I am moving data down through my max link, 12mbit (on 2 links, 2x5mbit in the queues).
I use PPTP as carrier protocol.
And the 450G is quite challenged CPU wise, showing encryption being one of the reasons.
Now, I am very reluctant to make away with encryption, but maybe PPTP is not optimal 
I am upgrading my links soon from 2x6mbit/0,5mbit to 2x6Mbit/,25mbit + 2x6mbit/6mbit. OBVIOUSLY when all the traffic there flows I will run more CPU - projection shows around 160% capacity, which is a problem. Even the 1100AH will be taxes, but there I can mitigate it (it is not that critial there anyhow, should move CPU to around 50%).
Anyone a sensible advice? Would L2TP be more efficient? Or do I have to move the 450G out and some Atom based Mikrotik in to handle the load? I am just asking - maybe another VPN mechanism is a lot more CPU efficient.