Canon 5D Mk II shoots only at 2.1 FPS burst instead of 3.9 claimed, why?

I've timed my 5D Mk II at 2.1 FPS during burst shooting, which is way lower than 3.9 FPS promised by Canon.
This time does not depend on image quality / size so I assume memory card is not a limiting factor. I have AF disabled. Tried both with and without live view. Noise reduction is disabled.
What am I missing?

P. S. My speed measurement is very accurate - performed by recording and analyzing shutter sound.

Answer

The maximum frame rates are just that - maximum frame rates. There are several things that will reduce the maximum frame rate.

• High ISO The higher the ISO you have selected, the slower the frame rate will be.


• Noise Reduction the stronger the in-camera noise reduction selected, the slower the frame rate will be.


• AI Servo Mode If you are using AI Servo AF mode the camera will refocus between each frame.


• In low light Most Canon User Manuals say "The continuous shooting speed might also decrease indoors and under low light"


• Various other options Other options that increase the amount of in-camera processing per shot can cause the frame rate to slow. Peripheral Illumination Correction, Auto Lighting Optimizer, Highlight Tone Priority, etc.


• Low Battery Charge When the charge state of the camera's battery is low it can reduce the frame rate.


• Increased Detail Increased amounts of fine detail in the scene, such as a field with grass.


• Slow Shutter Speed Selecting a very slow shutter speed will reduce the maximum frame rate. For example, if the shutter is staying open for 3/4 second per exposure, you won't get faster than about one frame per second.


• Defective Memory Card Although this usually manifests itself by reducing the number of shots before your buffer fills and increases the time it takes the buffer to clear and the frame rate while the buffer is full, it can also affect initial frame rate.

Since your question eliminates many of these issues, unless you are using a very high ISO or a shutter speed slower than about 1/250 second it sounds like there may be something mechanically wrong with your camera.