Comment 255 for bug 125970

Regarding the slowdown encountered when scrolling pages with fixed elements (item 1 in the prior comment) ONLY:

This is apparently still an "issue" on my machine. Here's how I tested:
With smooth scrolling off;
Scrolling by holding down the Down Arrow key on the keyboard;
On a system w/ WinXP, Athlon 64 X2 6000+, nVidia GeForce 8800GTS;
Under build Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1;

Using attachment 133309 w/ static menu, (0 fixed elements) CPU usage is ~0%.
Using attachment 133309 w/ fixed menu, (1 fixed element) CPU usage caps at ~4%.
Using attachment 170761 (100 fixed elements) CPU usage caps at ~11%.

It is my opinion (@Gérard, Dimitrios) that the above testcases are simplified and demonstrate this problem effectively. I do not believe that there is a need for any additional testcases on THIS BUG. Some later testcases make this behavior more apparent by combining it with other problems (attachment 326909), but are not really necessary.

I can't say whether the above performance numbers are acceptable or unacceptable; it only demonstrates that the engine must do much more work to render pages with fixed elements.

This isn't a problem for me since I have a very powerful computer, but (particularly when exacerbated by the other bugs mentioned above) may be a problem for older or less capable computers like the EeePC or iPhone.