Here is an interesting analysis of VC pitch decks that were hosted on the DocSend slide hosting/viewing platform. They aggregated data (anonymously) about what slides were included, how long the slides were viewed, how many meetings it takes to close the round etc.
Investors spend on average 03:44 minutes on a pitch deck, and 12% of them does so on mobile devices. In the second screen shot you see what this means per slide: 10 to 25 seconds. That is all you have to get your message across.
I do not completely agree that the ranking shown here implies how important slides are. Slides with financials, competitors, and team bios on them take more time to digest.
You can see the full results of the survey here.