Yet, oddly, Dashcam Viewer is smooth as silk on my Win7 Ultimate x64 first-gen, 2007 i5 Dell Precision M4500 laptop, playing A119S 1080p@60fps that are hosted on a 1gbps network machine.
It does have an SSD system drive, but 'only' 8GB of RAM, CPU is i5 M520 2.4GHz. GPU is Nvidia Quadro FX880M.
DV will not run smoothly on the old clunker that hosts the dashcam files. That has the same W7U x64 OS and a Core2Quad 2.4GHz CPU, 8GB RAM, AMD Firepro 2270 GPU, but the system disk is a fairly slow HDD.
DV runs fine in spurts, slowing down and evidently emptying a buffer faster than it can refill it. This is cyclical, every few seconds. To me that it works fine every few seconds suggests that the machine can easily cope with decoding and display, but DV's video library is spooling data to a cache on disk. The first machine has SSD and copes fine, the stuttery one has a cruddy old Samsung HDD from about 2005, which does not. It's an old dogsbody that basically does NAS and backup server duties. I expect that if I gave it a system SSD it would run DV just fine.
EDIT: In fact I have a 3rd old W7U x64 machine of similar spec, only a 3.3GHz E6800 CoreDuo CPU 8GB RAM and SSD. I will do a test install on that and see how DV copes.