Speed tip — Raw Cache
Lightroom and Photoshop use the same ACR, Adobe Camera Raw module to process the photo into the Develop module. The difference between Lightroom and Photoshop is that the ACR is built-in Lightroom, while in Photoshop, it's a stand alone application.
When Lightroom reads the a photo for the first time, Lightroom adds that photo to the Camera Raw Cache. The problem is that there is only so much cache. By default it's only 1 Gig in size, it's not that many photos, so the older photos are removed from the cache to make space for the newer photos.
When you load the photo in the Develop module, Lightroom will look first look for that photo in the Raw Cache. If it's not there it will render the photo, save the photo in the Raw Cache and then display the display the photo.
You can change the size of the Raw Cache to whatever you can or have space, you can also change the location of it:
Edit → Preference → File Handling → Camera Raw Cache Settings
Lightroom: Camera Raw Cache Settings
Tags: Dam | Lightroom | Lightroom-How-To | Technical