The vast majority of routers you will only get 20-30mbps of vpn throughput. They are being limited by the CPU. To get really fast vpn performance you are better off buying a small pc and putting a extra nic card in it and run one of the free linux router images. It does not have to be anything real special unless you are trying to get gigabit speeds.
What is needed is called crypto accelerator. Most modern CPU support a special set of instructions called aes-ni. Intel started putting this in some I3 chipsets and later.
Router this feature is very rare on consumer models. Commercial equipment is out of even your $800 price point in most cases.
There is a cpu for routers that I know has this feature made by broadcom...