I think better question was "why Nelsons were so fast"
Nelsons were more ore less direct response to Colorado and Nagato.
After WNT Britain did not need extra battlecruisers. Only ships of that class were in Japan – 4 Kongos, and Britain had Hood, Repeir, Refit and Tiger.
Hood stronger than any of Kongo ( first unmodernized version ), Tiger on pair.
Armour of Repeir and Refit were to be upgraded, so after that upgrade no worse than Kongo.
Problem was with those 16” battleships in USA and Japan.
On paper, no existing British battleship could much them ( now I don’t want to start discussion about difference between “paper” and “real” capabilities, so say only “paper” capabilities )
New 16” battleships were needed.
British knew that max speed of Colorado is ~21 knots.
They “knew” that max speed of Nagato was ~21 knots, possibly 22, not more.
So 23.5 for Nelsons guarantee some speed advantage over those most important “enemies”.
Clearly in WWII it was a bit too slow, but that time Nelsons were not so “new”.
In case of reconstruction.
Many things were considered.
After remedy of most urgent early problems, priority of upgrading ships were:
1. Add deck armour forward of the citadel ( done on Nelson, not on Rodney )
2. Add light AA armament ( done on both )
3. Upgrade AA fire control ( done on Nelson, not on Rodney )
4. Add extension of belt armour to protect against diving shells ( not done ).
And than in very late 30thies it was real need to repair they electrical systems.
This alone was expected to take ~9 months if not more. So there were some conceptions of larger reconstruction including new machinery.
New machinery was considered to gain ~2 knots extra speed. So max speed with new machinery will be something like 26 knots.
In my opinion more was impossible.
Power/Speed curve shows clearly that in low speeds Nelsons required very low power. Lowest of all British WWII era battleships.
But over ~20 knots it started to rise very sharply. Somewhere around 23 knots Nelsons required more power than KGV to gain the same speed.
Curve ends at this point, but tendency is quite visible. Possibly over 26 knots it should be nearly vertical, so even very large increase of power will result marginal increase in speed ( 2 shaft machinery limited possibility of waste increase of power anyway, and putting more shafts was problematic at best )
So in time, Nelsons were not so “slow”.