Here is the first video by Prodatron showing SymbOS on Amstrad CPC with AMSDAP + MSX Graphics9000.
At 2mn45 of the video, a video is running in SymbOS. The following words are from Prodatron : 16 colour bitmaps require the double amount of data compared to a 4 colour bitmap of the same size. The blitter inside the V9990 is doing a lot of things to speed up the graphic output. But it can't help so much when just pumping bitmap data from CPU memory to the screen. A video with the same resolution will have about half the framerate in 16 colours compared to 4 colours. So video output is not the best example for demonstrating the power of the GFX9000 (aka =CPC Powergraph).