I don't really now what you mean when you say laggy. I run my Lightroom on an Athlon dual core (2.4 GHz and 2 gigs of DDR RAM). What I can say is:
1) Lightroom is no speed daemon
2) When importing a set of files Lightroom starts a lot of background processing, mostly creating preview images.
My experience is that it is best to import the files, have a break, and then do development. Beta 4.1 on my hardware seems quite usable to me.
Does anyone else find LR lags from time to time? I especially notice it in the "Develop" mode when adjusting the sliders. I'm running decent hardware (P4 3.2 with 4-Gigs of PC3200 DDR, dual 7800 SATA HD's in Raid-0 configuration and a GeForce 6800 GPU (running in Win XP Pro).
I tried turning everything off in the background and LR was still giving my machine fits. This is the only program that lags on my machine. Could it be because I load all the photos in Library before developing them, or is it just because its a Beta version?