The new template downloads slowly!or
The new template scrolls irregularly.or even
It froze my browser!
Well, these are all real observations, and explainable ones at that.
Here's the "wallpaper" from my Nitecruzr Buzz. This is a full size (1800 x 1200) x 24 bit color photo. As rendered, my photo management program tells me that it uses 6.5M of RAM.
Traditional "wallpaper" would be maybe a 20 x 20 px snippet, at 256 colors (carefully muted), and tiled. Now we have color photos, 1800 x 1200 x 224, that fill the browser window. That will take a few seconds to download, and some power to display.
But, there's more.
The new templates feature the blog content housed in a transparent layer, so you can see the background beneath the content. As you scroll through the blog content, the background is displayed, in its glory. Your computer has to redraw the background, and the blog content, constantly, as you scroll. That takes computing power.
Watch the CPU trace in your system monitor, sometime, while you scroll your new blog. I just did. It's not pretty.
So consider the demographics of your blog readership, a bit. If your readers tend to use older computers, and slow Internet service (dialup even!), you might do well to look for a background that's lighter in resource use.