It finally crossed my mind that the issue might be with the driver and not the OS per se. So I went back to an XP system and upgraded it to 258.96. Immediately the SPDBV solid 3D performance became unusable. Have now tested a few driver releases in between and so far the last good one (fast solid 3D) was the default Microsoft one (126.96.36.19913 9/17/2008) and the earliest bad one (horribly slow solid 3D) was 191.87 from (2/9/2010). Both of these have the new Nvidia Display control panel. Will narrow it down further an then file another bug report. Hopefully Nvidia will fix it then.
: It turned out to be not that simple. Downgrading from 258.96 to any other driver resulted in that other driver being just as screwed up as 258.96 (or worse). Upgrading in steps from 178.13 (the driver XP pulled off the Microsoft server following a device manager uninstall of the graphics card and a reboot) worked pretty well up to 197.03, then went way down hill at 258.96. However, upgrading directly from 178.13 to 197.03 resulted in a broken driver, as did installing 197.03 immediately after uninstalling the graphics card. Ugh. I will start another thread in the driver section and place a link to it from here.