The reload rate is slow compared to other T7 BB's because her salvo weight is enormous. You can use this fun website to quickly compare ships and their characteristics: https://shiptool.st/
Then also check the shell ballistics for that big AP salvo...
You can see that California has a combination of several factors - 2nd highest AP salvo weight, accuracy, range, and ballistics, that all make her an excellent sniper. The tradeoff is that she is slow and that she has a long reload. The reload is slightly worse than Florida and Sinop, though her shells are heavier than Florida's and there is no comparing range/accuracy to Sinop.
If you're not a fan of the heavy-broadside / Sniper style that the American "Standard" Battleships bring, you might enjoy Gneisnau, Prinz Heinrich, or Renown'44 as they are much better suited to close-quarter slugfests.