The Kafers were engaged in a series of bloody nuclear wars before the city-state of Laresi finally won and unified the entire Kafer race into the Associative ca. 1850 AD. The Kafers discovered stutterwarp in 1910, and had working starships around 1920, but they lacked the computing power to produce a stutterwarp capable of high-warp. If I may quote the Kafer SB:
As it was, stutterwarp gave them the stars. Early Kafer stutterwarp was woefully inefficient,
with warp efficiencies of .005 or less. Improvement in the design eventually allowed warp efficiencies
of .01, but further improvements were impossible. The reason lay in the weakness of
Kafer computer technology.
Any stutterwarp requires a high level of computer technology to run, adjust, calibrate, and
maintain the drive. Human reactions are too slow to control or anticipate shifts or anomalies
in the drive fields, and far too slow to make the necessary nanosecond-to-nanosecond calculations
necessary to establish each microjump's position and quantum state. When it is realized
that a stutterwarp with a drive efficiency of 1.0 is travelling at a rate of something like 100 meters
every nanosecond, that each nanosecond's travel requires calculations of field strength, density,
gravitational moment and vector, and position to establish the ship's quantum reality, the need
for fast and accurate computational power becomes obvious.
Truly efficient stutterwarp drives require electronic monitoring systems and system-dedicated
computer control capable of trillions of individual calculations per second. The single weakness
of Kafer systems was not so much the hardware—Kafer computers were bulky and clumsy but
could have handled the task—but in the programming. Kafer programming techniques had lagged
behind the development of the necessary hardware by a century or more.
Kafer stutterwarp theory was sound, but Kafer computer programming was not up to the task.
Early explorations among the stars, then, still required long periods of time...more than 550
days to the nearest star, longer to systems more distant. But some 30 years after they began
exploring near-Serpentean space, they encountered Ylii.
They first encountered a Ylii outpost in 1950 AD and in 1960 attacked the outpost, killing 90% of the Ylii (system not stated, but probably BK+7 5675, 3 "jumps" from GS). In their explorations they discovered a proper colony some time later. They expected a Ylii attack but none came. Instead the Ylii welcomed the Kafers, which confused them. After about 50 years of sporadic diplomacy, the Kafers attacked that colony. We are probably around 2020 at this point.
After this, the Kafers figured out Ylii drives were far better. Eventually a third colony was found, and then a fourth etc. The Kafer started raiding these colonies for slaves rather than nuking them from orbit. However, the third Ylii colony (at O'Neil 723) was only conquered about 100 years ago and they did nuke it from orbit.
This suggests the practice of taking Ylii slaves and the jump to "modern" stutterwarp has happened in the last century. The Ylii worlds were:
BK-2 0075 (M2): 3 "jumps" "south" of GS, second Ylii system found and attacked
HC+ 3 1919 (M7): 4, adjacent to BK-2 0075
Oneil 723 (K2): 5 jumps, adjacent to HC+3 1919, conquered ca. 2200 after being nuked.
DK+21 6825 (K2): 6 jumps from GS, a spur from Oneil 723
HC+25 1902 (K2): 6 jumps from GS, and centre of the "Ylii cluster." Only recently conquered.
DK+32 2390 (M2): Still Ylii
DK-33 1023 (K2): Still Ylii
SS-27 6854: Ylii homeworld
Thus, I think modern Kafer stutterwarp comes later than Human stutterwarp.