Final answer:
A child exhibiting the phonological process of gliding and understanding they did not mean "wed" when attempting to say "red" shows that linguistic performance lags behind linguistic competence.
Step-by-step explanation:
If a child's language exhibits the phonological process of gliding and they say [wɛd] for "red", yet understand that they did not mean "wed," this demonstrates that linguistic performance lags behind linguistic competence.
The child knows the word and its meaning but cannot yet physically produce the correct sounds. This separation between competence and performance indicates that while the child has the semantic and phonological knowledge (competence), they are still developing the motor skills and practice required to articulate the words correctly (performance).