I am afraid it just might be so. A 23 Mb file can be enormous, especially if it has lots of component instances. The graphics card is just one part of SU:s display performance. It takes care of mostly the textures and shadows in the model, and the "geometry" (faces and edges) is handled by the CPU (or half of modern dual-core CPUs, as SU is single threaded like most modeling software). So an older 2.8 GHz CPU might even be faster in CAD than new sub-2GHz dualcores. SU uses OpenGL, not DirectX.

I think that Nvidia Quadro GPU:s are generally OK, but I have been a bit underwhelmed by the SU performance of the lower end of the price range (I have used the 550 and 1400 cards), and I think the high-end is overpriced. I see no big difference with the performance of the GeForce FX 5700 Go in my luggable, and the QuadroFX 1400 in my computer at work.

However, you should check that

the graphics driver software is up to date and the correct version OpenGL Hardware Acceleration is enabled in the Window menu>Preferences>OpenGL dialog Antialiasing is set to Application-controlled in your graphics card settings

To make SU go faster you can, for instance

turn off textures and shadows hide parts of your model that you are currently not working with use 2D cutouts (with alpha-channel transparency) for trees and people instead of 3D components replace taxing 3D components with simple placeholder objects, and reload them when your model is finished.

Anssi