Bah... I can only post to so many forums at a time! (and when I have time... *sigh*)
I'll just repost what I said on the mameworld.info forums for whomever's benefit, or to help testing:
I finally have Mamewah running pretty decent on my new machine, so I could do some testing for you...
Game/Emu: Any
List size: 100+ (to allow for testing/scrolling)
Missing artwork: Manually removed between 1 to 10 artworks of various types (I only have artwork1 and artwork2 being used...sorry!) All artwork was located on the same drive as Mamewah, and was not shared over a network.
Testing: The main emu/list I used had a full set of matching art1 and art2 files. The emu had it's own layout.lay and other files. I took out artwork around the letters T-V. I started with one, and worked my way (quickly) to 10, gauging roughly the amount of 'slowdown' or 'lag' in the scrolling as compared to other sets with full artwork. All my artwork is in PNG format, and in 640x480 (or similar 4:3-ish proportion) resolution. No transparent pictures are used.
Results: Starting from 0-A, just removing one artwork from t-v resulted in choppy/laggy scrolling. It got vaguely worse as I removed them. Strangely enough, once I actually got down to the bottom of the list (and the games with missing artwork were scrolled through), I could scroll back through that same list with no slowdown until I got around halfway through.
Fix: I couldn't figure out why my other lists were fine...even ones with missing artwork, until I realized that in *those* lists/emus (all my emus have emu-specific layout.lay files in their emuname folders) I have art.png and art1.png files as 'No screenshot' and 'no title' filler pieces. The only thing I can think of is that Mamewah looks for pic files and tries to buffer them, and by not having artwork in your 'snaps' location, or a supplemental generic pic in your config\emuname folder (art.xxx), it gets stuck trying to look for a matching picture.
Unfortunately, i'm not sure exactly why the scrolling-lag would be so pronounced at the top of the list for missing artwork in the bottom of the list.
Either way, try making non-transparent filler pics for your layout, and name them accordingly. Example: (how it's set up on my machine, might work differently for yours)
\config\emuname folder has:
layout.lay (emu-specific layout)
main.png (emu-specific main/background pic)
logo.png (emu-specific logo for my logo position)
art.png (generic "no screenshot" for my artwork1 position)
art1.png (generic "no title" for my artwork2 position)
So in my Vectrex folder, this would show up as:
\config\Vectrex
layout.lay
main.png (nifty Vectrex background)
logo.png (nifty Vectrex logo)
art.png (blurred-by-photoshop Vectrex logo with big letters of "No Screenshot Available")
art2.png (Blurred-by-photoshop Vectrex logo with big letters of "No Title Shot Available")
With those two files, the slowdown/lag disappeared for me. If you have one layout that you use for all your emus/programs, you can put the 'art' pics straight in your \config folder.
IIRC, somewhere in one of the mamewah docs (or maybe on a forum?) I saw that mamewah has problems with transparencies, so keep that in mind for your pictures.
...And don't forget that animated gifs are locked (won't animate) unless you get the registration code/reg key from Minwah by donating (sometimes repeatedly) on his site.