When shooting with my 7D with the drive set to High Burst (or standard burst for that matter), I notice that if I'm shooting a darker subject or a subject that's mostly in shadow, at the same shutter speed, my burst speed slows down dramatically compared to if I were to shoot a brighter subject. No settings change.
For example, shooting a subject right next to a window will have a higher burst rate than when I pan away from the window until the light fall off is just outside the frame. Both would be shot at the same shutter speed and aperture (ie. 1/100 @ F2.8 ISO 1000). Is this normal?
I also tried to shoot where the darker subject would be two stops below the brighter subject and then compensating by changing just the ISO, leaving the shutter and aperture the same, and the same result. The darker subject makes the shutter have a slower burst rate.
Yes, it is normal for the 7D to lose FPS in low light. Apparently the exposure processing is slow (cheap). There have been several threads on the subject.
It is standard metering logic on both the 7D and 1D4. It is discussed in the white papers and noted that as ambient light decreases the frame rate will drop. Read it and weep but it is a fact of life. This was also the case in the last generation of 1D3 and 1Ds3 but it seems much less pronounced, frankly almost unnoticable. EB-1's comment is interesting but inaccurate, the exposure processing is not slow. Digic IV is approximately 4 times faster then Digic III.
davenfl wrote:
It is standard metering logic on both the 7D and 1D4. It is discussed in the white papers and noted that as ambient light decreases the frame rate will drop. Read it and weep but it is a fact of life. This was also the case in the last generation of 1D3 and 1Ds3 but it seems much less pronounced, frankly almost unnoticable. EB-1's comment is interesting but inaccurate, the exposure processing is not slow. Digic IV is approximately 4 times faster then Digic III.
Dave
I'm pretty sure they don't use Digic to handle metering. I think the main CPU does that.
It's all integrated into a system, I didn't want to go into the exact specifics of the chip design and function, I didn't think that was necessary. I have to catch myself because non techs like to refer to the whole integrated computer system as Digic IV which isn't exactly accurate. The problem starts with the metering triggering a domino effect throughout the "system" to slow down in low light. Suffice to say the processing power of the 7D and 1D4 integrated system is in excess of 4 times the previous generation.
So the processing power is 4x greater, but the camera slows down more in low light. How does that slow down the frame rate? Does it take longer to read or reset the newer sensors?
EB-1 wrote:
So the processing power is 4x greater, but the camera slows down more in low light. How does that slow down the frame rate? Does it take longer to read or reset the newer sensors?
EBH
The firmware was redesigned to do this in the 7D and 1D4. The metering logic trips logic in the firmware to slow the frame rate, the camera isn't slowing down because of any issues or processing problems. Canon is doing this by design. Read the white papers, it is there is plain english and black and white. It makes sense but not really logical when you set the camera to manual and it stills operates the same way rather then overriding the automatic functions which doesn't make sense.
The meetering should be uncoupled from the drive in manual mdoe when not using auto ISO for sure.
I cant see it being a big fix at all, hasnt affected me but it does seem stupid that a subsystem your not using affects perfomance in a mode that should ignore it.
There have been several threads on the subject.
I gave up reading all of the explanations after a while back then. 
How does that slow down the frame rate? Does it take longer to read or reset the newer sensors?