Seth Godin notices how - because of information overload - we have stopped reading/absorbing things with the full nuances, referring to “TL;DR”: too long, did not read.
You as a presentation designer need fight against this behaviour as well. Dense boring slides do not get attention, instead people apply their mental models and think they already understand what you are gong to say.
The obvious approach to this is to design visually attractive slides to grab people’s attention. But visually striking images is only half the work. Your story itself need to have interesting and unexpected turning points as well. An unexpected fact, an unexpected contradiction.
People do not really think something is too long: they think something is too boring to read: TB;DR.