When you look at card reviews, they will list a game and then a name like "DM-Antalus" or in today's AT 3.2 GHz review, "We used our old 1.29f build of the game with the classic demo "four" at High Quality defaults." These are "standard" demos, used by a number of sites over and over again.
The problem is when ATI or more recently nvidia "optimizes" their drivers to detect these specific demos and cut corners in their rendering based on knowing exactly how this one demo goes. The cut corners might not be noticeable in these specific demos but either
(a) when playing the game "for real" the cut corners show up as image-quality glitches
(b) the driver turns off the cut corners so the game runs fine, but slower than it did for the cheated demos.
You can tell a demo / timedemo is custom if the site says so, otherwise it's probably a canned demo and might not reflect the card's true performance when really playing the game.
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