Your slide is too adj
What's an adjective to indicate that there are too many words on a slide?
If I were to type about sixteen paragraphs describing what exactly this word really means, then that would be a very wordy explanation.
Crowded is another option, if you're just referring to the unavailability of white space.
This slide is too cluttered.
This slide is too busy.
This slide makes me feel claustrophobic.
I also like dense, which was proposed in a comment.
It's not specific to slides or PowerPoints, but you could use "verbose".
characterised by the use of many or too many words; wordy