If only one side had this tech, the war would be over immediately. They'd stroll through and eradicate their enemy.

For there to be two opposing sides with the tech someone would need to steal it from whatever side invented it, and then have a battle sort of like you describe.

There's no tactical reason to slow down time unless you're setting "sleeper" traps, have the last survivors of a war trying to outlast their enemies, or something like that. For battle you'd only want to speed up so you can outperform your enemy.

Speeding up like that has its own set of problems.

For a short battle where one side wanted to destroy or capture the other, it works. The time wielders would speed themselves up so that their opponents couldn't see them, probably go set off explosives to kill everyone or put everyone in restraints or something, and then restore themselves to "regular" time.

The amount of time they age while sped up would be maybe a few hours or if it's a giant army maybe a few days. They've essentially shortened their life in "real" time by however long that interval was.

If BOTH sides are trying to out-speed each other it gets dicey. Now instead of a short amount of time spent at hyperspeed you have a protracted battle with no way to predict an end-date. Trench warfare like WW1 or something like the invasion of Germany in WW2 that took years in real life now might take decades or centuries. You could find yourself in a situation where you've won the war but everyone who cared about the outcome has died of old age in the meantime.

Then you have to consider that if both sides have the same level of tech and/or there is a cap on how fast the tech can speed you up, there's no point in using it at all. You're just aging yourself faster than your universe for no gain.

If one side can go faster than the other, the battle takes no "real" time but from a practical standpoint the defenders could force the attackers to allow a lot of "real" time to pass as described above by forcing the use of this tool. It would probably be a pretty effective albeit costly deterrent.

Armies could also use the tech to construct rapid fortifications, train, make vehicles, etc. Again... an arms race here would be difficult to maintain unless you literally had parallel societies running at different speeds.

There's an original Star Trek episode (Wink of an Eye) that deals with this. The Enterprise encounters a race of people who are accelerated and are effectively powerless to stop them until Kirk and Spock accelerate themselves.

Once the crisis has been resolves, Spock stays accelerated so he can repair the ship at hyperspeed.

For there to be two opposing sides with the tech someone would need to steal it from whatever side invented it, and then have a battle sort of like you describe.

There's no tactical reason to slow down time unless you're setting "sleeper" traps, have the last survivors of a war trying to outlast their enemies, or something like that. For battle you'd only want to speed up so you can outperform your enemy.

Speeding up like that has its own set of problems.

For a short battle where one side wanted to destroy or capture the other, it works. The time wielders would speed themselves up so that their opponents couldn't see them, probably go set off explosives to kill everyone or put everyone in restraints or something, and then restore themselves to "regular" time.

The amount of time they age while sped up would be maybe a few hours or if it's a giant army maybe a few days. They've essentially shortened their life in "real" time by however long that interval was.

If BOTH sides are trying to out-speed each other it gets dicey. Now instead of a short amount of time spent at hyperspeed you have a protracted battle with no way to predict an end-date. Trench warfare like WW1 or something like the invasion of Germany in WW2 that took years in real life might take very little "real" time but when it's over everyone who survived went in at age 18 but came out decades later.

There's also the issue of how one supplies an army at hyperspeed. They'll still need to eat and drink, but they'll deplete the local food, water, and vegetation and they'd still be subject to real world time, unless you also speed that up. Something like food could maybe be grown at hyperspeed, but water is a finite resource that could get consumed very quickly.

Then you have to consider that if both sides have the same level of tech and/or there is a cap on how fast the tech can speed you up, there's no point in using it at all. You're just aging yourself faster than your universe for no gain.

If one side can go faster than the other, the battle takes no "real" time but from a practical standpoint the defenders could force the attackers to allow a lot of "real" time to pass as described above by forcing the use of this tool. It would probably be a pretty effective albeit costly deterrent.

Armies could also use the tech to construct rapid fortifications, train, make vehicles, etc. Again... an arms race here would be difficult to maintain unless you literally had parallel societies running at different speeds. Again though, they'd need to share the same natural resources and the same water table.

There's an original Star Trek episode (Wink of an Eye) that deals with this. The Enterprise encounters a race of people who are accelerated and are effectively powerless to stop them until Kirk and Spock accelerate themselves.

Once the crisis has been resolves, Spock stays accelerated so he can repair the ship at hyperspeed.