That's what I've found. I first noticed then when running my program on a lowly 266 MHz K6, and it was really, really slow, when commenting out various things I removed the sphere-with-texture backdrop and suddenly increased the speed 5 to 10 times. First I thought it the sphere complexity (though only CreateSphere(12) in resolution) but replacing the backdrop with a full-screen quad had the same problem. I then tried a half-size quad and the spped was back up to fast and smooth.
I raised this with Mustang in the atmosphere thread and he found similar slowdowns, but of course it was rendering a lot more pixels. So it could have been on this old GPU (ATi Rage I think) there was a dramatic fillrate bottleneck that isn't as present on newer GPUs. A quick test program shows increasing backdrop fill doesn't have this dramatic drop-off threshold I experienced on the slower PC, but has a proportional change in render speed relative to quad size, whether that's made of multiple quads or one big one.
It might not be an issue if you don't care to support anything older than maybe a GeForce 2/3.
I raised this with Mustang in the atmosphere thread and he found similar slowdowns, but of course it was rendering a lot more pixels. So it could have been on this old GPU (ATi Rage I think) there was a dramatic fillrate bottleneck that isn't as present on newer GPUs. A quick test program shows increasing backdrop fill doesn't have this dramatic drop-off threshold I experienced on the slower PC, but has a proportional change in render speed relative to quad size, whether that's made of multiple quads or one big one.
It might not be an issue if you don't care to support anything older than maybe a GeForce 2/3.