Update: Now that the official version of Lightroom 5 is out, I've rerun the speed test and I can confidently state that exports and previews build at identical speeds in Lightroom 4 and Lightroom 5. Original Post Below
I've taken the opportunity to run some benchmarking test to find out if Lightroom 5 beta is faster or slower than Lightroom 4, specifically rendering 1:1 previews and jpeg exports. For my speed tests, my computer is a 2012 (unibody, non-retina) Macbook Pro 15″ quad-core 2.3ghz i7, 16GB RAM, and 5400rpm hard drive. My raw image files are from a Canon 5D Mark III. Time displayed in seconds, shorter is better.
Rendering Previews were comparable but the exports were tremendously slower. I did not apply any adjustments to my files so I can't blame the new tools. I double checked my results. We can only hope that the final release of Lightroom 5 will be much faster when exporting images.
Another interesting feature of Adobe Lightroom 5 is the new Smart Previews. Smart Previews allow you to build 2540 pixel length previews that will work offline so you don't need to store all your files locally or carry around an external drive. You will need to build the Smart Previews before you take your original files offline. In my testing, Smart Previews build in a little over half the time as 1:1 previews, but remember at 2540 pixels on the long end, they would be half as long and a quarter of the area of the full 1:1 previews. The Canon 5d Mark III files are 5760 pixels on the long end.
How much space do you save? We're still running comparisons, but in a Lightroom 5 public beta test catalog containing 1100 raw files, the standard Previews.lrdata file (which contained 1:1 previews) took up 3.65GB of storage. We exported this same catalog of images using only Smart Previews and the resulting Smart Preview.lrdata file weighed in at a meager 420MB. The size of each Lightroom catalog (.lrcat) itself was essentially identical.
So Smart Previews would allow you to have about 8.5 times as many images locally versus raw images.