My Mersey class cruiser anachronistic design

The next design I tried was for a Mersey class 2nd Class cruiser (anachronistic, of course). This ship had mixed results, in that the speed is just 21 knots. The protection is so much better and I do not understand why. This ship has a twin 8in turret forward with a superfiring twin 6in turret above it. To the stern are two twin 6in turrets. There are twin 6in turrets on each beam. This would be a ship with a single stack. This is the Springsharp report:

Mersey, anachronistic version, Great Britain 2nd Class Cruiser laid down 1908 (Engine 1912)

Displacement:
 3,573 t light; 3,781 t standard; 4,470 t normal; 5,021 t full load

Dimensions: Length overall / water x beam x draught
 318.05 ft / 315.00 ft x 46.00 ft x 19.50 ft (normal load)
 96.94 m / 96.01 m x 14.02 m  x 5.94 m

Armament:
      2 - 8.00" / 203 mm guns (1x2 guns), 256.00lbs / 116.12kg shells, 1908 Model
   Breech loading guns in a turret (on a barbette)
   on centreline forward
      2 - 6.00" / 152 mm guns (1x2 guns), 108.00lbs / 48.99kg shells, 1908 Model
   Quick firing guns in a turret (on a barbette)
   on centreline forward, all raised guns - superfiring
      4 - 6.00" / 152 mm guns (2x2 guns), 108.00lbs / 48.99kg shells, 1908 Model
   Quick firing guns in turrets (on barbettes)
   on centreline, all aft, 1 raised mount - superfiring
      4 - 6.00" / 152 mm guns (2x2 guns), 108.00lbs / 48.99kg shells, 1908 Model
   Quick firing guns in turrets (on barbettes)
   on side, all amidships
 Weight of broadside 1,592 lbs / 722 kg
 Shells per gun, main battery: 120
 2 - 18.0" / 457.2 mm submerged torpedo tubes

Armour:
   - Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
 Main: 2.00" / 51 mm 2.00" / 51 mm  2.00" / 51 mm
 2nd: 2.00" / 51 mm 2.00" / 51 mm  2.00" / 51 mm
 3rd: 2.00" / 51 mm 2.00" / 51 mm  2.00" / 51 mm
 4th: 2.00" / 51 mm 2.00" / 51 mm  2.00" / 51 mm

   - Armour deck: 3.00" / 76 mm, Conning tower: 9.00" / 229 mm

Machinery:
 Coal fired boilers, complex reciprocating steam engines, 
 Direct drive, 2 shafts, 13,079 ihp / 9,757 Kw = 21.00 kts
 Range 6,500nm at 12.00 kts
 Bunker at max displacement = 1,240 tons (100% coal)

Complement:
 273 - 355

Cost:
 £0.413 million / $1.654 million

Distribution of weights at normal displacement:
 Armament: 199 tons, 4.5 %
 Armour: 839 tons, 18.8 %
    - Belts: 0 tons, 0.0 %
    - Torpedo bulkhead: 0 tons, 0.0 %
    - Armament: 223 tons, 5.0 %
    - Armour Deck: 563 tons, 12.6 %
    - Conning Tower: 53 tons, 1.2 %
 Machinery: 866 tons, 19.4 %
 Hull, fittings & equipment: 1,669 tons, 37.3 %
 Fuel, ammunition & stores: 897 tons, 20.1 %
 Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
 Survivability (Non-critical penetrating hits needed to sink ship):
   4,407 lbs / 1,999 Kg = 17.2 x 8.0 " / 203 mm shells or 1.0 torpedoes
 Stability (Unstable if below 1.00): 1.09
 Metacentric height 1.8 ft / 0.6 m
 Roll period: 14.2 seconds
 Steadiness - As gun platform (Average = 50 %): 54 %
   - Recoil effect (Restricted arc if above 1.00): 0.96
 Seaboat quality  (Average = 1.00): 1.22

Hull form characteristics:
 Hull has a flush deck
 Block coefficient: 0.554
 Length to Beam Ratio: 6.85 : 1
 'Natural speed' for length: 17.75 kts
 Power going to wave formation at top speed: 58 %
 Trim (Max stability = 0, Max steadiness = 100): 44
 Bow angle (Positive = bow angles forward): 3.00 degrees
 Stern overhang: 2.00 ft / 0.61 m
 Freeboard (% = measuring location as a percentage of overall length):
    - Stem:  20.00 ft / 6.10 m
    - Forecastle (20 %): 15.00 ft / 4.57 m
    - Mid (50 %):  15.00 ft / 4.57 m
    - Quarterdeck (15 %): 15.00 ft / 4.57 m
    - Stern:  15.00 ft / 4.57 m
    - Average freeboard: 15.40 ft / 4.69 m

Ship space, strength and comments:
 Space - Hull below water (magazines/engines, low = better): 102.2 %
  - Above water (accommodation/working, high = better): 106.2 %
 Waterplane Area: 10,148 Square feet or 943 Square metres
 Displacement factor (Displacement / loading): 107 %
 Structure weight / hull surface area: 97 lbs/sq ft or 476 Kg/sq metre
 Hull strength (Relative):
  - Cross-sectional: 0.87
  - Longitudinal: 3.35
  - Overall: 1.00
 Hull space for machinery, storage, compartmentation is adequate
 Room for accommodation and workspaces is adequate
 Good seaboat, rides out heavy weather easily