I found something about Sony SLT A77's viewfinder. It still repeats what @Itai said about varying refresh rates in different lightning conditions, but what I found interesting was what they said about lag in good light. It is hardly noticeable. A quote below (bolding done by me):
When not in continuous shooting mode, viewfinder responsiveness is a function of refresh rate; essentially the frame rate of the "live movie" that constitutes the viewfinder image. Specs for this are almost never published, in part because the refresh rate is usually a strong function of the amount of light falling on the camera's sensor. In dim lighting, the camera needs longer exposure times for each frame of the movie it's capturing and playing into the viewfinder display, setting an upper limit on the viewfinder's refresh rate. Indeed, as noted above, the Sony A77's refresh rate drops pretty dramatically when faced with very dim shooting conditions. (A trade off we're happy to accept, given that the resulting viewfinder image is much more visible than that from an optical viewfinder under similar circumstances.)
We don't have any way to measure the Sony A77's viewfinder refresh rate under bright lighting conditions, but our sense is that it's quite high: Motion was very smooth and fluid, and we had no sense of a delay or lag between the subject's motion and what we were seeing in the viewfinder. - source: Imaging Resource
Now, as there is no, and perhaps never will be, a reliable way to test electronic viewfinder display lag in varying conditions, I think this question is ready to be closed.
It is hard to say more than that there is some lag in dim light, and practically no lag with the best e-viewfinders in best conditions. Catching a moment that you have been waiting to photograph, your own human reaction time is the biggest lag there is.