I've tested it on a laptop, a 1200 MHz P3 with 512 MB of memory. It runs at approximately the same speed there, which is a bit surprising since the machine is slower, but on the other hand it has a smaller screen (1400x1040 vs 1600x1200) and a much smaller library.
I just tried using fewer fields (on the P4-3000). With only three fields (artist, name, duration) I can go through a screenful of songs in 6.5 seconds, while the standard setup requires 12 seconds and my own setup requires 16.5 seconds, so it's clearly related to the amount of data which is displayed.
CPU usage is at 100% when I do this, so it's not just slow to react; it's really doing something which is taking a lot of time.
When you scroll, are you using the down arrow? Why not use page down?
I'm not even scrolling. I'm moving through the files that are already visible on the screen! And of course I don't always want to move an entire screenful at a time, but that seemed like the easiest thing to measure. Even if I just want to move 10 files down in the list, it feels like molasses and I tend to start reaching for the mouse instead.
No Girder. I didn't think I was using any third party plugins but now I see that MC11 picked up the ones I had installed in MC10. I'll uninstall them one by one and report the results.
I already updated my entire library, and thumbnails were created for all files so that shouldn't be the issue here. Also, this happens even if I move through the same page of files / songs over and over again; it doesn't get better over time.