This will give them time to find any extra information they need, and also more time to practice. More timid students may want to meet with you again. Conferences should be more than 15 minutes long. Have the student give the speech two times. Take notes about clarity and style.
Some students are naturally good speakers.
Do not worry about them. Poor delivery usually is a sign of poor preparation, especially if they are reading the speech word for word. These students either need to practice more, do more research, or reorganize their speech. That is why the conference should be held several days before the actual presentation. Do they use gestures to emphasize, or for no apparent reason.