The following article contains spoilers from The Flash #795, on sale now from DC Comics.
The Fraction has quickly proven themselves to be one of the deadliest enemies the Flash Family has ever encountered. However, The Flash #795 (by Jeremy Adams, Roger Cruz, George Kambadais, Fernando Pasarin, Wellington Dias, Oclair Albert, Luis Guerrero, Matt Herms, and Rob Leigh) made it even more apparent when they were reporting back on their progress in stealing the planet's resources. Metahumans were also a resource they were harvesting, but they revealed that the kryptonians were giving them trouble, to which their commander stated "as always"
This indicates that the Fraction has a history of dealing with Kryptonians. A fact that isn't too surprising considering their full capabilities. Still, it makes them even more terrifying when one considers that this means they have gone up against multiple Kryptonians and won each time to get to this point.
The Flash's New Alien Enemies Are a Force to be Reckoned With
Part of what makes the Fraction such an insidious enemy is that they aren't really on Earth to conquer. Their arrival on Earth decimated the surrounding area, but that was only the opening salvo in their invasion. Using their speed, the Fraction sought to strip the planet of all its natural resources to fuel their empire's unsustainable expansion. Their efforts do not end at raw materials though as they also wish to collect metahumans.
The Flash Family has already seen that they have been rounding up heroes and villains alike to take back with them. Superman was among them until the speedsters liberated him, but he was still adjusting to the speed at which the Fraction was moving. As a result, Kryptonians are one of the few beings capable of having a chance at stopping the Fraction. This can be seen in the report given to the Fraction commander regarding their containment. The fact that they were giving the Fraction any sort of resistance is a testament to how strong they truly are. But the general unconcerned, even familiar response from the commander indicates that this is nothing new to them.
The Fraction Have Encounted (and Defeated) Superman's People
Now, while Kryptonians are a rare breed in the DCU these days, it is important to remember what the Fraction can do. Their speed force technology doesn't just give them greater speed, it also allows them to travel across time and space. In all likelihood, they have encountered various Kryptonians over the course of their many conflicts, be it from different universes or points in history. Each time, they have triumphed, as Superman's people likely did not have enough time to adjust to the increased rate of movement before the Fraction claimed them.
What's disturbing here is how it implies the Fraction are more than capable of handling Kryptonians. Granted, it may be because their abuse of the speed force makes it almost impossible to keep up with, but victory over a Kryptonian can come in many forms. The Fraction may be using the speed force to steal from a planet before its inhabitants can fight back. But they also have learned how to use their power to its fullest effect, wiping out any sort of resistance to their plans, no matter how powerful.