Pierre:

From your last 2 emails and tests as compared to what I see, I am thinking that the graphics board is the bottleneck.  Doing similar tests with the Clowns, as compared with your observations below, I am always getting close to 29.97 fps in either X11 or X11-OpenGL.  The reason I think it is probably your graphics board is because my laptop is not really a "work" computer but rather a "gaming" computer (it was an inexpensive AMD computer that has never, ever played a single game!) so I would imagine the graphics board is meant to be pretty good.

The results of these tests of the mpeg proxies tell me that with both
the X11-OpenGL driver and the X11 driver, using vdpau results in a very
slight reduction in the use of my CPU, but that this does not improve
the frame rate possible that these video drivers allow to display...

The above seems to indicate that the graphics board does not improve anything and you have plenty of CPU anyway, so you might as well use that.

X11 allows in all cases to display at least 29.97 frame/sec sources that
have been shot at this speed.

X11-OpenGL is always limited to a maximum of about 12 frames/sec.

These results are approximately true for all the types of media I
tested, whether DNxHD.mov, HDV (MPEG-2.m2t), AVC H264.mp4 or even
proxies in mpeg.mpeg.

Given these results, I don't really see the advantage of using
proxies... In any case, the video driver used will determine the
possible frame rate regardless of the type of media used.

I'm actually wondering if the constant frame rate limit of 12 frames/sec
provided by X11-OpenGL in my tests with 4 mixers, regardless of the
media type, doesn't actually indicate a bug somewhere or a limit
inherent in my equipment. But then how do you explain the best
throughput with X-11?

Instead of working with 29.97 fps media, I loaded Big Buck Bunny which is 60 frames per second.  And there may be something strange going on as Pierre indicated that I will have to test on a faster computer.  Because when I played this, like Pierre, it seems to limit it at always 30 fps whether I user X11 or OpenGL.  Then when I proxy it to 1/2, I thought I should have improved the frame rate but it too was at only 30 fps. 

I will have to do the tests on GG's computer to eliminate the possibility of a limitation / bug.  Phyllis