Which features are important in determining fluency of participants measured by weighted composite score?
This is a tentative answer.
One can try to determine first the relative importance of each feature (eg by Factor Analysis or Principal Component Analysis, ..)
Once the more significant features have been identified (or guessed perhaps), then one can try combinations of scores with these features.

Take simply the average as a composite score.
Do a linear regression and use the coefficients as the weights of the composite score.