Like most on here I've had slow times with LR3 and have a spec'd up 'puter... so I've done digging and come up with this thought process...
Firstly, it's nothing to do with the front end of our machines, CPU and RAM etc are fine, several brush strokes are easy for a CPU to do and don't need much RAM. What slows us down in to do with three things:
Writing this data back to the LR3 cache used in develop mode.
Adding data to the sidecar/xmp file
Some NVidia driver tweaks...
LR3 can have a BIG cache, so give it one, up to 200GB, preferably not on your start up drive and not on an external drive on the end of a USB 2.0 or Firewire 400. Put it on an internal 'scratch' drive with plenty of space on it. Read this wonderfully clear explanation http://lightroom-blog.com/2010/04/camera-raw-cache.html
So, keep LR3 cache off your start up drive, put it on a 2nd internal drive if possible or a drive connected by Firewire 800 or eSATA, now what about your ACTUAL files... where are they...? LR3 usues the cache to do the redraw etc... but it does write stuff within the DNG/RAW file, so when you open it up somewhere else it shows your adjustments, so the location of your files is important and if they are 'miles away' on a 5400rpm fairly full drive in again connect slowly this will contribute to additional speed loss. So put them inside your machine if you have space, using eSata or put them in an external air cooled box, again with faster firewire or eSATA, both are monumentally quicker than USB 2.0.
Thirdly, there have been speed improvements for nVidia graphics card users by tweaking the set up to run for 'max performance' in LR3 and set the card up for LR3 rather than gaming. A good clear tweak is here http://www.thejohnsonblog.com/2008/09/06/l...ia-performance/
Also here: http://www.flickr.com/groups/adobe_lightro...57607074073712/
For me I've almost eliminated the dreaded slowness by doing the following:
LR3 cache on 2nd internal 7200rpm drive connected by esata cable
LR3 cataloge's and images on big 2TB 7200rpm 3rd internal drive, esata connected
back up data stored on external box of drives
tips: 2nd internal drive is 500GB and has LR3 cache set at 200GB.. why not..? and has very little else on the drive, and is defragged weekly
defrag all drives weekly if you use a lot, also update video drivers and try the tweaks..
All windows Vista/XP x32 and x64 users... I know it takes afull day and is annoying but..... a 'format c' and re-installation of the OS is amazing... I hate to say it... but we know it's true.