Most people complain about Samsung after-sales service, which I would say is really quite bad. They stopped updating my phone (Galaxy Spica) at Android 2.1, because "it's old". It's just about a year old. And it doesn't want to provide proper graphics drivers and that is the main cause of any lag on the Galaxy Spica. I've installed OpenGL drivers on my phone and there is really a marked improvement in 3D stuff, especially the Gallery. But either way, phone manufacturers have to stop updating after some time, it's inevitable when they come up with new phones. But for Android, we still can rely on developers for updates.
I see alot of people recommending Galaxy S, it's a very good phone (its specs are really good - and Samsung makes just about the best screens for mobile phones) but I've heard that Samsung screwed it up (as they usually do) and the phone isn't performing as good as it can be. For the best optimised Android phones out of the box, I'd say go for HTC, they seem to know what they're doing better than Samsung (in terms of speed and interface - hey Touchwiz pales in comparison to HTC Sense, but maybe it's just me). But still, if you know how to (or are willing to find out how) root your phone and install custom ROMs made by developers, that's the best when you want speed.