I've worked in clinics that expected that we do 15-18 patient exams BEFORE NOON. That is too stressful and not enough time to do a thorough exam. Mostly, 30-45 minutes per exam allows enough time to do a comprehensive and thorough assessment for ob/gyn or general. Vascular and Echo might require more time but every facility has different expectations. You will be slow at first but do not get discouraged by that. Factors like different machines, patients body mass, children squirming or crying, and even gas are challenges that we all face. You can't work magic and get the perfect image every time but you can learn tips and tricks to optimize that image when you are faced with a difficult exam. Fast does not mean thorough. Also, you must pay attention to your own body and not harm yourself. Sonographers are prone to musculoskeletal issues if you aren't practicing ergonomics while scanning.
All that said, a great and full day could be 10-13 patients. 5 patients in a day is considered a slow day for me.