This might be Dave Wolverton's (AKA David Farland) 1989 novel On My Way to Paradise.
From the Amazon link above:
In a world of ever-worsening crisis, Angelo Osic is an anomaly: a man
who cares about others. One day he aids a stranger. . . and calls down
disaster, for the woman called Tamara is also a woman on the run, the
only human with the knowledge that will save Earth from the artificial
intelligences plotting to overthrow it.
Fleeing the assassins who seek
him as well as Tamara, Angelo seizes the only escape route available:
to sign on as a mercenary with the Japanese Motoki Corporation in its
genocidal war against the barbarian Yabajin. Jacked into training
machines that simulate warfare, Angelo "dies" a hundred times. . .and
is resurrected to fight again. In a world of death, he dreams only of
life-and the freedom to love once more.
The main character, Angelo Osic, is a doctor (of sorts, he's more of a pharmacist who uses nano tech "drugs") On the run after helping a mysterious woman, Osic joins a group of South- and Central-American "chimeras", genetically augmented superhumans created to fight an earlier war. The chimera mercenaries have been hired by a Japanese corporation to fight another Japanese corporation on a disputed colony planet. The book is divided in three parts - the events leading up to his time on the ship, his time training on the ship, and the events after landing. The Latino chimera mercenaries are kept separate from the rest of the Japanese crew and soldiers on the ship. His shipboard training time is curtailed when he's put in stasis for his involvement in a failed mutiny. The advanced training he missed includes battle drugs and mental disciplines to help soldiers think and move faster.
Here's the "slowing time with their minds":
Chapter 22:
“We’ve learned much,” the chimera said. “You were frozen before
training really began. You did not spend time studying your own
individual kinesthetics through the holographs, learning not to waste
a move, learning the spins and throws and drops necessary to defeat
the weapons of the Yabajin. You were still studying how to achieve
munen, the state of no mind, the state of the living corpse. You did
not progress into the higher mental states necessary for
battle—Instantaneity or Perfect Control. The great genius of the
samurai comes from their knowledge of these states of being—”
“Pardon me, but what do you mean by Instantaneity?”
The chimera looked at me thoughtfully. “Perhaps you have had a moment in your life, a moment of great fear when your life was in jeopardy, and time seemed to stop. I once lived such a moment. In an alley in Temuco during the riots a human came and put an ancient revolver in my face and pulled the trigger. The hammer was cocked, and when he pulled the trigger it began to fall. Time seemed to stop. A car passed on the street before me, and a woman was looking out the window and watching. I remember her face perfectly, her ruby lips shaped in a circle of surprise. I saw the steam rising from a sewer grate across the street, and a man in a store there was turning out the lights. I looked into the frightened eyes of the young man who planned to murder me and knew he could not be more than fifteen. And all this time the hammer was falling and I thought, ‘When the hammer drops, I die.’ So I reached up and put my finger in front of the hammer and it never dropped. That is Instantaneity, living life in a moment. It is a state of mind a warrior can learn to induce at will. It is one secret of the samurai. Beyond this lies Perfect Control—the ability to achieve a measured heartbeat, to stop one’s breathing, to put all muscles under voluntary control.”
