Unlike LightRoom and other image processors that convert the otherwise unviewable raw file into an RGB JPEG file, IJFR takes a different approach. Inside almost every RAW file there exists a JPEG file that was processed by the camera. Just like if you had shot JPEG instead of RAW. So when you are shooting in RAW only (the camera is actually shooting in JPEG+RAW, but rather than putting the JPEG where you can get at it, the only copy of the JPEG file is inside of the RAW file.
What IJFR does, is simply dig into the RAW file and extracts the entire JPEG file and saves a copy of it within the file system of your Mac or PC. So after processing by IJFR there are 2 JPEG files for each RAW. The one extracted by IJFR, and the original one already inside of the RAW file.
The time for IJFR to extract the JPEG file is a fraction of the time required to actually create a new JPEG file from scratch using the RAW data. The camera has already done the hard work of creating the JPEG. IJFR just has to make a copy of it for you to have instant access to it.