doom3 plays better on higher setting?

Question

can anyone explain why when i play doom3 on the lowest setting and res,it runs terrible ,but after a bit of tweaking i set it at medium quality and 1024x768 and its working great
this is even more amazing as the machine i am running it on is a pentium 3 866mhz ,512mbram,radeon 9250(128mb)
how does a game play better at a higher setting,when it plays rubbish on the lowest,and the radeon 9250 isnt the greatest card?

Answer 1

It says on the case of my Doom 3 that the minimum cpu requirement is P4 1.5GHz or Athlon XP1500+...
Are you sure you're not running it on your P4?
Fess up, you've mixed your machines up - too many late nights on Y! Answers.

I wonder if downgrading the resolution possibly puts some additional loading on the cpu - it has the overhead of conversion from the "natural" resolution? On a lower power cpu (and in this context the P3 qualifies) the cpu then becomes the bottleneck.
The 9250 is fairly recent and although not fast, I think it has full hardware support for directX9.0b.
So I'm speculating that even with medium quality 1024x768 it's the cpu that's limiting the system - but as it's not having to tweak the graphics it' actually has less to do.

It would be interesting to know how the 9250 compares to the 9550 you appear to have fried last week...