you are definitly not alone. These problems occur on specific systems and are well documented now.
Steinberg has given the following general reply on this issue:
viewtopic.php?f=196&t=53066&start=50 (and lots of others)
The graphic performance appears to be linked to the latest drivers' implementation, with some of the improvements made for gaming and 3D graphics causing real-time spikes with particular plug-ins.
These plug-ins all share common characteristics - they over-ride the default behaviour of the pointer when hovering on the controls, so it might be related to a particular GUI framework.
I'm no developer, so I forwarded the outcome of my tests to the proper people in-house.
They were actually already on it and the GUI handling is being revised - it is quite unlikely that the drivers will be written the "old way" in the future, so it must adapt to the way drivers are working now and will probably work in the near future.
By the way, during my tests the behaviour of Cubase 7/7.5 and Cubase 6/6.5 was the same concerning this.
But Cubase 6/6.5 only shows the average VST Performance, so the spikes and fluctuations might look like a normal fluctuation due to the load, especially with a lot of plug-in instances or working at low latencies.
When looking at the Cubase 7/7.5 real-time meter, it might trick into thinking that the performance is much different, but the only way to directly compare the two versions is by comparing the average meter only.
Actually there are lots and lots of post about this issue on the forum. A good starting point is by using the search function on top of the forum and type "asio spikes" or "graphic performance" as search terms. You will find lots and lots of different findings.
Some have found a temporarily solution by rolling back the graphics driver to an older version. Some have confirmed that even changing a OS mouse scheme to an inverted setting can be a solutiong. Others have posted that a complete reinstall solved the issue. Also online you can find similar warnings when using specific graphic related software.
Some graphic card tools like Ati Power Play and Nvidia Powermizer interfere with real-time audio, since they prioritize the graphic card performance over other processes in the system. A solution there could be to disable or uninstall these tools.
Hope this sets you a little on track of the problem you have.
Maybe SB should make a more general posting about this issue, since this is rather frequently asked.