I see you already got excellent answers, so I'll just add to this a bit.
All blades are less efficient when the tips approach supersonic. Be it turboprops or helicopters. These blades are unshielded and so are subject to several unknown forces - turbulence, wind shear f.ex. - that might not even be noticed by the flight crew. (There are of course also the centripetal forces from the blades itself, but that is already designed for.)
To eliminate this unknown environment, the turbofan has its fan blades shielded. The inlet design gouverns the air flow to it to a much more reliable speed and consistency. For thrust purposes you have the compressor compressing the air after the fan.
On the Concorde they designed a very special inlet just to slow the air down when cruising at supersonic speeds for this reason. I strongly encourage you to read the thread about the Concorde when you have time. It is located here in Tech Log and is truly awesome!
Last edited by MrSnuggles; 30th Jun 2015 at 14:26.
Reason: correcting for speed/thrust confusing