"That's pretty swift," said Bury's Guy, who was fifth in the individual 200m and sixth over 400m.
"In the (individual) 200 free final I got giddy, got excited and went for it. It doesn't work like that any more.
"Tonight I stuck to my game plan and it worked."
Britain took Olympic silver in the event behind a USA squad including the now-retired Michael Phelps in Rio, but were without a key member of the squad from 2015 and 2016 as Dan Wallace is serving a suspension following a drink-driving conviction.
Guy, who replaced Calum Jarvis following the morning heat swim, won individual 200m freestyle gold and 400m silver in 2015, but was disappointed with his individual performances at the Rio Olympics and again here.
Until Friday, when he first advanced to the 100m butterfly final behind the USA's Caeleb Dressel, who clocked 50.07, the fourth fastest time in history. Guy finished in 50.67.
Asked about his slow start to meets, Guy, who swapped Millfield for Bath after Rio, said: "Once the 400 is out of the way I can relax more. It's that burden on day one.
"The 100 fly, coming into it, hasn't meant anything. And I've got no pressure.
"What I am finding is I am more relaxed and swimming faster."
Guy even hinted at dropping the 400m freestyle from his schedule in future and adding the 200m butterfly.
Milne and Scott won Olympic silver in the relay, but Scott and Grainger had been switched out after the heat swim in Kazan, so all three won the world final for the first time.
Grainger, 22 from Sheffield, said: "I made a promise to myself in 2015 that I was going to do everything in my power not to be in that position again.
"Standing on that podium was absolutely fantastic, having a team like this is brilliant.
"When James touched at the finish, I thought I almost took Duncan out running in for the hug."
