1. Get a new computer. Okay, I know, this one's unfair. But it's true. Like any piece of software, Lightroom has minimal system requirements. Get your system up to snuff, or preferably well beyond those minimal requirements, and your software will zip along nicely. Upgrades to hard drives, RAM and processor are the most obvious contributors to hardware speed.
2. Let the software help you. Run regular operating system updates and make sure your version of Lightroom is up to date as well. Bug fixes can occasionally help speed the software, as can closing programs you're not using. This lets the computer concentrate its processing power in Lightroom. These first two tips are good advice for any program, though, so let's now get a little more Lightroom-specific.
3. Free hard drive space. With a larger hard drive you're likely to have more free space. But, whatever drive you have, the fuller it gets the slower the program will run. Lightroom requires a bare minimum of 1GB of free drive space, but you'll want at least 20% of the drive that holds the application, image catalog and previews to be free in order for Lightroom to run quickly. (Store those previews and catalogs on the same hard drive to keep LR running quickly, too.)
4. Run in 64-bit mode. Check that Lightroom is running in 64-bit mode—to access the most RAM possible and speed workflow—by checking the title bar in Windows for "x64", or checking File Information on Lightroom in the Applications photo of Mac OSX. If it's not, be sure the "Open in 32-bit mode" box is unchecked.
5. Be deliberate with Lightroom previews. Keep standard size previews as small as possible, and set larger 1:1 previews to render automatically on import. Alternatively, you can choose to manually render these previews after import but before you set about navigating through a take. If Lightroom is tasked with rendering on the fly, you'll find yourself waiting for previews to load. That's why rendering automatically on import or manually prior to browsing will speed up your workflow.
6. Turn off "Auto Write XMP." Adjustments made to metadata, keywords and other image adjustments are stored in an XMP file that by default is automatically and regularly saved regularly while you work. This is convenient for people who switch regularly between Lightroom and other editing programs like Bridge or Camera Raw. But, if you're strictly a Lightroom user, turning off Auto Write XMP—accessed in the Catalog Settings preference pane—will speed up the software. Plus, you'll still get the benefits of XMP updates whenever you output or print photos from Lightroom.
7. Export in smaller batches. One of the quirks of the way Lightroom 4 allocates your computer's resources is that it won't consume all of the available processing power when it's doing a batch import or export. What that means is that if you're trying to export 300 JPEGs from RAW files, for instance, Lightroom is only going to allocate about one-third of the available processing power to the export so that you can continue working on other things while the export proceeds in the background. This is great—unless you're in a hurry. If you simply want those 300 JPEGs as soon as possible, you'll have better luck by exporting the first 100 in a batch, then the next 100, and finally the last 100. Three batch exports, run simultaneously, will process out considerably faster than a single batch of 300.
8. Optimize the catalog by backing it up regularly. I set my catalog to back up on closing weekly. Not only is this helpful to keep files safe, but since the catalog is automatically optimized immediately following each backup—as long as you have the appropriate checkbox checked—backing up also serves to speed up the software. If you want to optimize more often, backup the catalog more often. While it takes a bit to backup, it can run in the background while you're doing other things, and it definitely keeps things running smoothly.