Perhaps 2048 hits a limit on your hardware and it falls back to software. You don't need to scale up to a power of two for the capture though since pix_snap2tex uses subtexturing or rectangle textures when available. Even at powers of two the 2048x768 is 2048x1024 which is the exact same number of pixels as the two 1024x1024.
yes i know that it would do that, but i did not get the motionblur effect when not using power of 2 snaps sizes and it also needs to be textured on a square.
one effect is also that the closer the alpha is to 1 the more the blur starts to behave like running ink.
the "real" effect i only achieved with square gemwins, pw2 snap sizes and fullscreen squares.
some tests for 1600*600 you can see at randomseed.org/view/bunt/ they are rendered with a alpha 1 - which gives this inky effect. in a patch where everything has square dimension this would look very different - like single buffering. it looks quite funky - but is not intended :-) .
if one takes the alpha lower the motion blur works really well though.
best
erich