The Barranquilla's (ex-Poseidon) speed increase is seen as nessassary if she's to compete with the other South American capital units. The 3.5 knot speed increase isn't really spectacular when you look at the Italian rebuilds of the Cavour's/Doria's, but to be fair they didn't retain their Q turrets.
The Poseidon basically set the speed standards in the Atlantean fleet for the period she was built up untill the end of the war when Melampus was built. To understand things better heres a list of Atlantean ships worth noting:
BB Eumelos, 1931, 28.18 kts
BB Memnon, 1929, 28.23 kts
BB Melampus,1919, 26.50 kts
BC Vengeance, 1918, 31.00 kts
BB Tyrrhenia,1913, 22.50 kts
BB Atlas,1911, 22.50 kts
BB Poseidon, 1910, 22.50 kts
BC Azaes, 1909/10, 27.1 knots
BB Mneseus, 1908, 21.5 knots
AC Lyra, 1908, 25 knots
BC Arcturus, 1907, 25.5 knots
AC Defence, 1907, 22 knots
BB Calessus, 1906, 21 knots
AC Cleito, 1906, 22 knots
AC Neptune, 1905, 22 knots
PD Etamin, 1905, 16 knots
PD Tyrrhenia, 1904, 16 knots
AC Callixtus, 1904, 22 knots
Highlighted are ships that made significant leaps in terms of capability's.