I have ben using ADW ever since I had the X10. It enables you to handle the home screens like 2.1 would. It gives you 5 screens instead of 3 as the normal default, which I find are more than enough. There are 3 things that it does, that makes it a must have on my phone:
1. I can resize items on my home screen and I can place them easier due to the resize, something the default UI doesn't do.
2. It also allows you to make Groups in the Drawer. With the Groups, you can choose what apps you want to show in your Drawer and which you do not. This enables me to get rid of the Bloat Ware. It doesn't delete it off the phone, but since I am not rooted, this is the next best thing (Thanks Rosered for pointing this one out for me).
3. One of the slickest features are the Themes you can use with ADW. Makes the phone look like a totally different phone and there are a lot of Themes to choose from, with people making more everyday!
So, I don't know about any lagg or slowness that is caused by ADW, however, I did find that all of those apps out there that monitor all of your system resources and RAM and , etc, etc, etc.. do slow down your phone! So much so that I deleted all, but one, off my phone as I was seeing lagg and choppiness. Since I removed them, phone runs great. I just think 1.6 really doesn't handle allocations to well and that is a nice thing to see in 2.1... Providing AT&T doesn't do more damage, lol..............