Let's figure out why 50 minutes is so difficult. Well, the first few minutes are occupied with housekeeping, announcements, collecting or returning homework, and students coming in late. (The last is something to work on.) So 45 minutes at best. Any review takes up another 5 minutes. (I do this once per week in my 100-level class.) So as not to bore students to tears, I try to craft learning exercises, which take up 10 minutes at the very least.* Add or substitute one video clip with any discussion whatsoever (5 minutes). For example, today we covered psychotic disorders in Abnormal Psychology, and I worked in some Ryan Gosling. Effective, but time-consuming.
That's 20-30 minutes of actual lecture. How many slides can you get through in 30 minutes? I can get through 10, if it's a really good day (i.e., if students have read and there aren't any tangential questions). How many do I have prepared for each lecture? Minimum is 12, mean is 14. Compounded over a week, that's almost a full lecture behind.
So what goes wrong? Well, most students don't read, so we waste time with me waiting for them to answer my questions. (Sometimes they don't even seem to have opinions about concepts like identity or friendship, which is beyond me.) Maybe I should pick out only one or two main concepts, and cover those in detail. Maybe I should give more quizzes to ensure that students are reading, so that I don't feel any pressure to review everything important in class. Something needs to change; my midterm evaluations were quite positive, but it seems as though we've gone downhill since then. Though I'm not yet sure how much is me and how much is middle-of-the-semester laziness.
Based on several sources of recommendation (e.g., word of mouth, student feedback, this recent article about using PowerPoint in the classroom) I've started breaking up even non-wordy slides to make sure that each one is visible and useful. My "lecture" style relies heavily on eliciting responses from students; I naturally pause at least once every 2-3 minutes to ask students for factual information (from the readings) or their opinions about concepts. What keeps my prep time down is that I don't plan out these questions in advance. Keeps the lecture conversational, but likely wastes a lot of valuable time.

*I admit that sometimes I dispense with this.