Not at all, there's 3MB of VRAM on board. That's enough to do 720p@24bit and 1600x1200@8bit which was a bit shy of the ragged edge of 16bit requirements IIRC. If memory serves, 3MB will support 4K resolution in single bit mode.
You might just barely be talking 16bit at only 1980x1050. Bit depth calculator's not handy. No NuBus card can match the "speed" of onboard video for pedestrian purposes like gaming frame rates at relatively low resolutions because of bandwidth limitations. NuBus cards are for pushing massive amounts of pixels in graphics apps, displaying huge spreadsheets or providing for a workable multi-window desktop.