How do you know your config was "pristine"?
I work with Cisco products every day, and have for over a decade. I'm very familiar with IOS.
They had connectivity problems and some sites were slow to come up. Yes, latency was part of it. The router had various overflows and buffer related issues during high usage. This was awhile ago so I didn't exactly save the error messages nor remember my CPU usage. I did some tweaking with the buffers, one-minute and incomplete parameters which helped, but it was just easier to put in a real firewall. It also helped to offload the VPN.
Eight years ago I used a 2610 for a home "firewall + VPN" and it was painful. I also had 4 offices use a 2610 in the same way, each with an Internet T1. A Sonicwall SOHO2 at that time was noticeably snappier. But the 2610 only had a 40Mhz CPU and 10Mb/half ethernet IIRC. It's not the stated PPS that slows it down, it's something else. My guess is CPU. (And these had the AIM-VPN/BP)
I'm not sure why don't agree with me about this. It's a router, not a firewall. Sure there is overlap, and probably not noticeable on most broadband connections. I imagine you would agree that it would be silly to replace an ASA5580 with a 7204VXR as the corporate firewall to use an extreme example.
Now ask yourself why.
.