back in Photoshop 7 there was a plug-in called "Enable Async I/O" which made disk operations (for writing to and reading from the scratch disk, and maybe with your files) faster. When you had that plug-in enabled (which it was by default), you saw an asterisk next to the efficiency percentage.
I think that maybe they built the plug-in into the program at some point, but didn't remove the asterisk, since I also see that asterisk.