Okay... some numbers. (tl;dr version == "big fat web pages with hundreds of tiny graphics are baaaad".)
I pulled up
http://soundcloud.com/tracks on my developer workstation here at work and enabled resource tracking. This is a hefty quad-core processor PC with 8gb of RAM. It took 16.80 seconds to load, and 15.63 seconds of that are images. 95% of those images are hundreds of tiny little avatar files that are choking the page out.
Using Chrome with the Developer Tools and enabling resource tracking, that page weighs in at over 2mb. I don't know if you know web development, but any page that's loading 2mb up front like that is gonna lag like a fat pig after Thanksgiving dinner.
The issue is the site's poor design choice in loading hundreds of stupid avatars letting people comment on this or that loop in someone's track. This is an example of a web site that would overload the iPad just because of bad design.
So, I loaded the same URL up in Safari on my iPad over my wifi network. Took about a full minute to load the entire page. That being said, once the entire page loaded (until the blue bar was gone, etc), I had no trouble at all playing any tracks. There was no skipping, missing, etc, and I could even see the silly comments go by as different users commented on the song (I listened to the entire "Come Together (A. Skillz Remix)" that was at the top of the page).
To be fair, I did make sure to shut all other browser windows and apps down to give it as fair of a shot as possible (I'm running iOS 4.2GMa).
You'll find other sites like this one (I believe listed in that other thread about RAM) where the site is just too poorly designed to play nicely on an iPad. That's not necessarily the iPad's fault. Sure, everyone wants more RAM. But for what it does I'm happy so far.
But no wonder Soundcloud sessions play so much better in the Korg iMS-20 app. They don't have to deal with the website itself, heh heh.