I'm afraid I don't understand the finer details and inner workings of DSfix, but I did several tests nonetheless. From what I've gathered, the default render (ie internal) resolution is 1024x768. It doesnâ€™t mater if you set another display value (like 1920x1080), itâ€™ll just stretch the image because internally itâ€™s still rendering 1024x768.
Running the game at default settings = 15fps, meanwhile running default + unlockfps = 25+, a considerable gain. Whatever it does isn't linked to the resolution, at least not directly. In another test, downscaling the internal resolution to 800x600 boosted the perfomance at the cost of image quality and even more so when unlockfps was enabled. Setting the priority to High for DATA.exe in task manager helps a little too, just not as much as unlockfps.
Check the Dsfix ini file, the author made some interesting comments there.