... at least 1.10.0.29 seems to have "restored" us the missing performance; the difference between the same spots in the main grid and the preview grid are now statistically irrelevant.
For the record, the new hardware setup routine changed my settings a bit: it automatically raised drawing distance from 64 to 128, turned shiny & bumpmaps off, as well as the avatar vertex program, aand had as lights "sun and moon" only, and AGP was unchecked. I changed those to reflect better my "usual" settings — 64 as drawing distance, AGP checked, avatar vertex program on, shiny + bumpmaps, and lights to "nearby" sources. The end result: a performance equivalent to the one on the main grid.
Sadly, some options are now unselectable in my case, namely, avatar rendering is now selectable to "normal" only (I wonder why). Ah well. I can only hope that this will be addressed in 1.10.1 or something like that (I guess I'll survive without those nifty features for a few weeks...).
I found a tiny bug when activating looped animations through the Gestures and crossing sim borders (scripted attachments that activate looped animations are not affected) — which I have reported — and have been seeing the "flickering" of some prims (they never get rendered at all, but by repeatedly crossing sim borders back and from, you can see that they are there!). This last bug is pretty much random, I can't repeat it. Sometimes prims will not be rendered at all, and sometimes they won't have any problem; if I manage to understand exactly what is going on and how to reproduce the bug, I'll be filing it as well.