If this is the case (and such situations are frighteningly common) the main problem is that slides probably contain way too much text in the first place. Try convincing these individuals to explore forms of graphical expression. In other words, ideally there should be VERY LITTLE text and far more pictures, video, meaningful animations, simple diagrams, etc.).

If your presenters still insist that slides must contain text, place one idea per slide and simplify the text to at most three words per slide (usually one or two words work well, at approximately 100-point size). This design strategy is effective for the audience, and at the same time gives presenters a clue as to what they are supposed to be talking about. Now they are forced to actually communicate with the audience, rather than read. Slides should never diminish a speaker's communicative bond with listeners.