Latency has never been an issue for me on my Wersi. Probably the reason Wersi utilizes a fast CPU and decent amount of RAM. In addition OAS 7 does run a custom Scope card as its sound engine which has its own processors as well, therefore minimizing taxing of the PC's CPU.
Be advised... All VST's aren't created equal and some can be major CPU hogs taxing even the fastest CPU's. This is why you don't get crazy and try and run too many of these VST's in your system. Just because you have a VST host that supports 16 or more VST's, don't be foolish enough to believe you're going to run that many at once and not have issues, latency inclusive.
With the right asio drivers lattency is acceptable, still there but not noticable under windows..
However if you want to have real time processing, and thats what you want in a system like this, you either want to use VxWorks (Most embeded systems use this) or a real time enable Linux Kernel. Since under VxWorks you would never be able to run VST's all that leaves us with is Linux or MAC OSX (which is nothing but a shell on top of a Unix kernel) and those VSt's that run natively (Not those that require WINE)
What i meant to say, that you can not garranty low lattency under windows when there is a high system load, you can however guarrantee them under a Linux/Unix kernel that is patched for real time processing.