OpenGL is the hardware acceleration library for Android. For an application like the browser, there can be two implementations of the "drawing" code for the application. If you wanted to draw some text on the screen, you could use either a bunch of standard text widgets, or you could render the text like a game would - with OpenGL. OpenGL lets you do things that a game would want to do, like scroll the text, rotate it, scale it, etc.
The OpenGL implementation of the browser would be much faster, and lets the device offload a lot of the graphical elements to the GPU.
Perhaps the problem here is that the keyboard (not OpenGL) is being composited on top of the OpenGL browser. That compositing implementation may be borked, or may be done in the CPU rather than the GPU.
So: Who's fault is it? Hard to say. It could be Google's implementation of compositing, or it could be the hardware vendors' (Samsung or Nvidia) implementation of OpenGL for their hardware.
I wonder if other OpenGL apps do the same thing. Someone find a game or graphic intensive application that also uses the stock keyboard and see if you get lag there, as well.