"Can I ask for an example?"

If you mean an example of how SDCC is inefficient I'm afraid not, I've never used it.

I would imagine, though, as it has been ported to the 8051 it may fail to take advantage of features such as bit addressable RAM or register bank switching.

It may use a stack based parameter passing scheme which would be very restrictive on the 8051 if implemented on the hardware stack and very inefficient if implemented in XDATA.

It may not provide local variable / function parameter overlaying in DATA space.

It may not take advantage of local calls/jumps.

These are only guesses really, maybe someone who has compared the code with that generated by Keil, for instance, would like to give some concrete examples.