The Revit Clinic blog has been posting an ongoing series of 10 tips for XXX. The latest installment is for Linking Files. Item number 6 bears repeating...echoing.
6. Central file or local file link?
When linking a Revit project file into a host project, ensure the link was last saved as a central file. Links that are considered local files will check for a related central file as they are loaded. This can cause a performance hit which can be avoided by ensuring all links are central files.
It is interesting that this is stated clearly for once. There are two threads (at least) at AUGI that suggest considering the reverse, that linking local files instead is better. It is always helpful when the Factory comes out and says something specific like this, as long as it is correct. ~smiling~