There's an important distinction to make between Color QuickDraw performance and raw bandwidth.
The way Nubus video cards accelerate the desktop experience is by handling QuickDraw instructions in place of the CPU, thereby allowing the CPU to immediately parse the next instruction while the video processor on the Nubus card goes about completing the draw task. In cases where the instructions are relatively straightforward to understand but take a longer time to actually carry-out, the performance is considerably greater. Scrolling through a full-screen finder window in 24-bit color is a good example of this: The CPU only needs to tell the Nubus card about the lowest bit of picture, the one that appeared from the bottom when you scrolled down- otherwise, the Nubus card just needs to take the rest of the window (which is already live in the card's VRAM) and move it the correct number of pixels higher. Since the Nubus card's controller is optimized for video tasks and has a very high bandwidth connection to the card's onboard VRAM, this can be completed much more quickly than a 68040 makes the same change to the Quadra's internal VRAM. As a bonus, the 68040 was able to go back to other computational tasks immediately after delegating the command to the Nubus card, further enhancing the speed of operation.
Where things get interesting is when the screen is being drawn with mostly unique information. Since there's no elegant way to utilize QuickDraw commands to draw a complex image, the 68040 sends a stream of small instructions, essentially manually going through each pixel and relaying its proper value. The Nubus controller can still translate this data to its VRAM with little issue, but the bandwidth shared between all Nubus devices is only 10 or 20MHz, compared to the 25, 33, or 40MHz dedicated connection between the 68040 and its onboard VRAM. As a result, the built-in video is noticeably faster at handling motion video type content.
As a quick test, you can open the 'Jigsaw Puzzle' app, generate a new puzzle with large pieces, and set your monitor(s) to 'Millions of colors.' Piece the puzzle together without letting it snap into the background, and you'll be able to drag the world-map around the frame in real-time.
The redraw movement is visibly smoother on my Quadra 950's built-in-video than the second screen on my Radius LeMans GT Nubus card, which is significantly faster than the 950 in QuickDraw tasks.
In general, most games of the period used 640x480x8bpp to minimize the drawing overhead of full-frames anyhow, so the lack of Nubus bandwidth shouldn't make anything unplayable, but don't expect video to be universally faster even if the card is said to be faster than the built-in video.