I have been using OpenCV's VideoCapture class to decode several H.264 camera streams simultaneously in a custom video viewer application. Unfortunately, when I upgraded OpenCV 2.4.6 to 2.4.9 my client reported a large slow down in the framerate at which the viewer shows frames. The framerates are roughly half of what they used to be (i.e., 30fps is now 15fps viewer speed).
I have since built FFmpeg from source myself, and am seeing very similar slow downs with my own build of FFmpeg 2.3.3 (as well as git head) (vs. 2.0.2, which is present in the OpenCV 2.4.9) using OpenCV's FFmpeg wrapper implementation.
I was wondering if anyone on the OpenCV team happened to have custom build flags for 2.4.6, or if they are the same as is shown in the GitHub readme? Was libx264 possibly included in some of the previous builds; I understand that libx264 is a higher performance H.264 codec, so maybe that accounted for the speed decrease? Has anyone else noticed this slow down? It may not occur when reading a single stream, but it is definitely noticeable when decoding many.