okay, a radeon hd 6670 is more than enough for smooth rendering.
Except without using WPF I can't believe that it can be slower than on other (older) systems.
I don't mean that you require
vmware, it's only a fact that applications doesn't run in vm as well as without using vm machines.
So my conclusion is similar with yours yet, if it's running smoothly with your other pc's (using the same setup,compiled as release version, not debug, same tiff file).
So I would try it on one more Win7 x64 and if it's running well, there must be something wrong with dev machine, but that's quite hard to say, can't have a look at your system configuration and your hdd.
Your code seems to work, I had a look over it and the only thing IMHO would change is using the
because of right Disposing() of the streams.
Please give a feedback to us, it's an interesting question for me