I was loving Mountain Lion till DP3 and now with DP4, the sluggish UI of Lion has returned. Apple probably realised that they have to hold back some performance to help dumb ****s to upgrade.
id say that its a bug. For me the only laggy part is when we switch spaces. other than that mission control is better and the scrolling is FANTASTIC. id just wait for the GM!
I noticed the same thing on my mid 2009 macbook pro, everything is laggy and it wasn't like this in previous previews. Let's wait for the final release