I am confused about why the repeated use of short sentences is read quickly when there's a period at the end of every sentence that should make the reader stop for a second or two every time?
Short sentences are short. Simple. They don't tend to be that complicated. They're easy to understand. Yes, the period is something that makes people stop. But when you have many separate phrases that are easily swallowed, the reader doesn't have to spend too much time trying to comprehend them. They read easily and therefore quickly.
Personally, I would say that long sentences are a bit slower to read because they are more complicated, and the reader needs to track more words before completing one idea - if one sentence is considered one idea - like this sentence - especially when complicated punctuation is implemented and/or a lot of commas, so if I were reading this sentence without being the one to write it, I would have trouble following exactly what was being said and would need to take a bit more time to fix that.
Also, for me, my brain sort of skips over periods. Not actually, but I just register them as breakers between sentences. You always need a period at the end of your sentence. They're predictable. I pause more in my mind when encountering a comma, because they're deliberate tools that authors can use with a reason, and their purpose is to give a pause. I don't necessarily think periods slow readers down that much - but maybe I'm alone on this opinion.
No matter what, I've heard that it's not a good idea to have a bunch of short sentences in a row or a bunch of long ones. You want variety, or else the writing becomes mechanical or super confusing. Hope this helped at least a little bit, and good luck!
It's not short sentences per se that speed up reading, but simple grammatical (and logical) structure. This can appear in long sentences, and it may in fact read a bit faster than short sentences. (you don't actually stop for a few seconds at every period though, unless you failed to understand the previous sentence. Either way, the same stopping will still occur when the sentence doesn't end, perhaps later, and by then it may take even longer to figure it out)
Anyway, grammatically complex sentences are long by default. Whether you have to backtrack consciously what some pronoun refers to, which part of the sentence continues after a comma, or piece together the intended sequence of events, it's a lot easier to write bad long sentences than short ones without trying.
Excessive filler words, and duplicated information, may actually speed up the pace at which you read, but in total they still take time to comprehend. Simply by making the sentences they appear in longer, they too tend to be more common in long sentences.
For these reasons, longer sentences tend to read slower.
Usually people use shorter sentences to build tension. People often read it out faster to make it even more tensiony (don't question my sources). If you want to use short sentences and not have it read aloud faster, put a really long sentence in the middle. I can guarantee people will then read it slower after that.
I used mostly short sentences in a novel draft once. It seemed right. Mostly. It was easy to read, but something seemed off. I couldn't tell what it was in the first place.
But after a while I realized: part of the tension lingering above the whole story, above every scene and conversation came from the short sentences. Because somehow I tended to synch my breath to the sentences. Even with reading in my mind, it felt like taking a breath at every period.
It was an effect that really served the story, but now I tend to vary the length of my sentences more to avoid a feeling like in this Video.
Because it's not so much about how long it takes you to read the paragraph, nor the length of the pause between sentences that gives the impression of speed.
It's the burst. The start and stop, the chop-and-change. It jumps about; darting from place to place. Your reach the finish, now. Then it happens again.
Compare a car with a housefly: You sit in your car, you drive along the road at 30mph / 50kmph, and you don't exactly feel like you're going particularly fast. A housefly only moves at 5mph / 8kmph - but it jerks about, constantly changing direction, so it seems fast. That's basically a light jog.
A football player running down the pitch doesn't give the same impression of speed as one jinking left-and-right, trying to trick their way past a defender - even though they are probably moving much faster.