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Telling us what you average right now doesn't say anything. A DIY won't work miracles on your time unless your current cube is really bad. But if your current cube really is bad, or you want a cube that you can adjust to your liking, a DIY is worth looking into.
Also, you should try a silicon based spray lubricant. Works wonders on cubes.
DIYs and store bought cubes can both produce good times. DIYs may give you a little faster of a time, but not by much. Also like shelley said, use silicone, it is much better and you don't have to clean the black gunk that builds up. I always think that it is more about the cuber rather then the cube, but that's just me.