Screen flash – yes, that is completely normal. Unlike LED displays, E-Ink technology doesn't constantly refresh. That's part of why it's so much easier on the eyes. What that means is that when you change what's displayed, the screen has to get rid of the previous page before it can display something else. That's most commonly seen as a negative image screen flash.

Slow wireless – also completely normal. Think about any browsing you may do on your cell phone. Since the unit's computer isn't dedicated primarily to that function, and it's using cell phone connectivity instead of cable internet or broadband, it doesn't perform it as fast as a "real" computer does. It's also a lot smaller, so doesn't have as much in the way of resources to devote to it. The Kindle's web browser is considered "experimental;" it's not what the device is meant to be used for, so they didn't put a lot into making that aspect of it really good.

Hope that helps.