A slide gets five seconds before an audience decides whether to keep reading it or look back at the speaker. Testing against that budget is the cheapest quality check there is, and almost nobody runs it, because it needs a second person.
The arithmetic sets the target. At about 200 words a minute of silent reading, a five-second glance covers 16.67 words. A slide of 247 words is 14.82 glances deep. Give a reader five seconds and they see roughly one fourteenth of it, with no way to know which fourteenth mattered. That's a lot of writing for nobody. A slide of 46 words is 2.76 glances. That's still more than five seconds, but it's inside the range where a skimmer reads the title and the one highlighted figure.
How do you run the 5 second test?
The protocol has four steps and takes two minutes per slide. Show the slide for five seconds, on a timer, to somebody who hasn't seen it. Hide it. Ask them to write down the one thing the slide says. Compare what they wrote with the message you intended, and score it right or wrong, with no partial credit. It feels a little awkward the first time you ask a colleague. It gets easier.
Run on a cohort of 24 readers, the wall-of-text slide in Worked example 2 was read correctly by 5, which is 20.83%. Its rebuilt version was read correctly by 21, or 87.50%. That's a gain of 66.67 percentage points, and 4.2 times as many readers got the message. (The tally comes from a synthetic training cohort built for the example, and no published experiment stands behind it. The protocol is the part that transfers, and your own numbers will differ.)
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What does the result tell you?
The result tells you which kind of fix you need. If a slide fails with a clear title and a scattered body, the problem is typographic: the hierarchy isn't directing the eye. If it fails with a clean layout, the problem is editorial. There are too many words, or there's no single message, and no amount of alignment will fix it. Run the test before you choose a repair, and you'll save the forty minutes the analyst in the opening spent on colour.