German hardware site PC Games Hardware claims multi-GPU solutions from AMD, NVIDIA and SiS all suffer from an issue they describe as micro stuttering. After reading countless messages about very small but noticeable delays in games with multi-GPU systems the site went out the investigate the issue and they conclude it's indeed a big problem:
Here the disclosed problem starts: multi-GPU systems often do not succeed in displaying regular distributed frames; it comes to large time intervals between individual images - "micro stuttering" - despite supposedly fluid frame rates. One example: frame 1 is followed by frame 2 after 10 milliseconds, which corresponds to a cadence of 100 Fps. Frame 3, however, is to be seen only 40 milliseconds afterwards, followed by frame 4 after 10 ms.
The result is disastrous: The subjective frame rate of a multi-GPU system is in this extreme case below a single card, although the displayed Fps are higher. This behavior is primarily critical at low frame rates, such as the aforementioned 30 Fps. The less Fps, the greater the possible intervals between them and a regular distribution gets even more important.
The following graph visualises the first 30 frames from a Need for Speed Pro Street race with one card and a CrossFire system. The single GPU shows a nice straight line while the line of the CrossFire system has small ripples that symbolize the in-game delays that many users complain about:
You can check out the full article over here, the site also has a video that illustrates the problem. AMD and NVIDIA are aware of the problem and will try to address the problem with a driver update.