Forget about the first thing you hear from most rookies. The speed of the game in the NFL is going to have little to do with whether former Southern Utah defensive end James Cowser makes the Raiders’ roster. He already has shown he can get by offensive tackles at this week’s minicamp.

What’s blown away the undrafted rookie is how much time and energy the veterans put in at the team’s facility in Alameda.

“They just work sooo hard,” Cowser said.

Khalil Mack had 15 sacks last year, and he’s constantly drenched in sweat this offseason.

“I was really impressed with the work ethic,” Cowser said. “You see all these big names, like Mack and (Bruce) Irvin, and so you watch them. They are really impressive.”

Cowser is making a good first impression as well.

He went undrafted despite being the all-time Football Championship Subdivision leader in sacks (42.5). Cowser also had an FCS-record 80 tackles for a loss, topping former Idaho State and likely future Pro Football Hall of Famer Jared Allen’s mark.

At6-foot-3, 248 pounds, Cowser wasn’t drafted. He was downgraded by some because of the level of competition at Southern Utah and his age (25), but not by Raiders head coach Jack Del Rio.

“Cowser ... was productive in college,” Del Rio said. “I really had a high grade of him, a high evaluation of him coming out. He’s just come in here and worked really hard.

“He’s been impressive in the fact that he’s a good learner, he understands what we’re asking of him, what we want him to do, what we want him to be able to do, whether it’s special teams or defense as his role. He’s very conscientious.”

As are his new teammates, who aren’t shy about giving out advice. Cowser also worked out with defensive tackle Justin Ellis during the offseason, as they share an agent.

“From little things to overall, they are always in my ear,” Cowser said. “Just trying to make the Raiders better. ... I’m just trying to learn as many positions as I can.”

Cowser couldn’t be happier after the initial, fleeting “disappointment” over not getting picked in the NFL draft.

“I turned it off as teams started calling to sign me toward the end,” Cowser said. “It’s not that big of a deal, really. It’s about how you play on the field, not where you were picked.”

Fifteen teams made offers, and Cowser hit it off with Raiders defensive assistant Sam Anno.

“It felt right,” Cowser said. “He kept saying, ‘You’re a good fit, come play with us.’ I had also been impressed with the coaches at the NFL combine. It’s all about the work you put in.”

And he is ready to put in the work, knowing sacks won’t come as easy in the NFL as they did in the Big Sky Conference.

“The drops are different, guys are bigger, they set up differently,” said Cowser, who is getting a lot of work at outside linebacker as well. “I’m just getting a feel for that. There are a lot more levels of rushing, not just going around the quarterback, like I could get away with in college.

“You have to be more headsy about it, using counters and your hands more.”

Cowser also will have to improve his strength and taking on players at the point of attack.

“The plan is to work hard and get better fast,” he said.

The other plan is to eat a lot of Chinese food. Cowser went on a mission to Hong Kong in 2009 on behalf of the Church of Latter-day Saints, and won’t miss the Chinese restaurants in Cedar City, Utah.

Oakland Chinatown restaurants will be seeing a lot of a long-haired guy in a Raiders shirt. Going over his notes from coaches and teammates, smiling ear to ear.

“Not getting drafted was a bummer for about five minutes,” Cowser said. “If you’re dealt a bad hand, that’s what you’re going to have to play, anyway. So make the most of it.

“I couldn’t be happier being on the Raiders and in Oakland.”

Vic Tafur is a San Francisco Chronicle staff writer. Email: vtafur@sfchronicle.com Twitter: VicTafur