RIO DE JANEIRO — Another night, another world record shattered.

Stanford incoming freshman Katie Ledecky will be taking a nice token to her dorm room on the Farm. A gold medal, earned Sunday night in the 400-meter freestyle, in a swim in which she absolutely destroyed the world record.

Of course, by the end of these Olympics, she might have so many medals she could hand them out as welcome gifts to her suitemates.

On Sunday, Ledecky shattered her own property. She set the 400 freestyle world record two years ago in the Pan Pacific Championships when she was 17.

After her historic swim, Ledecky looked up at her time — 3:56.46, almost two seconds faster than her previous record — smiled and raised her fist. And then she turned to hug the other swimmers, the silver and bronze medalists who came in almost five seconds behind her.

“Pure happiness,” Ledecky said of her feelings.

Ledecky, a 19-year-old from Bethesda, Md., set an Olympic record in her heat earlier in the day. Which is one of the reasons she powered through her last 50 meters.

“It felt like a similar swim to this morning,” she said. “I knew I had a lot left at the end this morning.”

Ledecky’s feat came 24 hours after Hungarian Katinka Hosszu destroyed the 400-meter individual medley world record Saturday night. That was also a swim that bettered the world record by about two seconds. As soon as Hosszu’s time was noted, the whispers started. The eyebrows were raised. The doubt was cast.

Not so with Ledecky. Is that because she’s an American and NBC controls the narrative? Or is it because there’s a difference?

Are other fast swimmers cheaters and Americans just awesome freaks of nature?

Debate among yourselves. Sadly, this is part of the drill when watching the modern Olympics, and the Russian doping scandal has only fueled the ongoing skepticism. The Russian swimmers were booed by the crowd at the pool Sunday night.

There is indeed a difference. Hosszu is 27, in her fourth Olympics, and is getting only faster. That’s reminiscent of Barry Bonds getting really good at hitting home runs in his later 30s.

Ledecky was a phenom in London at age 15, when she won gold in the 800-meter freestyle, and she has been getting steadily faster. Her coach, Bruce Gemmell, says she has “once-in-a-generation greatness.”

Ledecky, who won’t turn 20 until March, consistently swims the 400 freestyle in under four minutes, having set the world record three times. Her time Sunday was more than seven seconds faster than the world record set by Janet Evans in Seoul in 1988. That record stood for 18 years.

Ledecky’s time would have set the record and won gold in the men’s event in the 1972 Olympics (the current men’s record is 3:32.25).

She and Gemmell came up with a plan after the 2013 World Championships in Barcelona. They set out her goals for the quadrennial leading to Rio, and hitting 3:56 was the mark.

On Saturday night, Ledecky won silver anchoring the freestyle 4x100 relay. She is the favorite to repeat — well, dominate — the 800-meter free. She can swim fast over short distances and fast over long distances. She’s fast.

Ledecky is in her prime. How long she’ll keep improving remains to be seen. We’ve already seen 2012’s golden girl, Missy Franklin, struggle to stay on top. Now 21, she has gotten slower over the past four years, which is typical.

Franklin’s summer of dominance came in London. Ledecky’s is coming in Rio. She is expected to be the face of the Games, one that NBC will package for all of its prime-time coverage. And she’s pulling other swimmers along with her, buoyed by what is called “The Katie Effect.”

“Everyone’s chasing her,” said American Leah Smith, who gutted through the pain of the fast race to win the bronze medal. “It’s really exciting to hear the crowd going insane, and you can kind of pretend it’s for you even though you know it’s for Katie.”

Ledecky deferred her enrollment in Stanford by a year to train for the Olympics. She doesn’t know what she will study but will leave herself open. That’s refreshing; Ledecky probably has enough goals in the pool and could use a little freedom on land.

“I’m very excited to get on campus in September,” she said before the Olympics started. “That’s a new chapter. Right now, I’m finishing up this chapter.”

This chapter certainly will include more medals. And possibly more world records.

Why, Ledecky was asked, are so many swimming records already falling in Rio?

“I think because it’s the Olympics,” she said. “Everyone wants to swim their best time at the Olympics.”

Maybe it’s just that simple.

Ann Killion is a San Francisco Chronicle columnist. Email: akillion@sfchronicle.com Twitter: @annkillion