Between turns, the game has to calculate every other players turns, which is a lot of thinking to do. A faster CPU and more RAM help this a lot, but unfortunately that's ultimately just the nature of the game and even super high end computers get slowdown and lag between turns.
The main issue is how long it takes between turns, if you're getting huge times between turns then I suggest upgrading PC parts (CPU and RAM) or looking at one of the mods which restricts the amount of workers a civ can have as that's one of the main culprits for slowdown between turns.
But yeah unfortunately you will never be completely rid of this - all civ games are like this it's just the nature of the game.
Edit: Also try playing with less AI players (this includes City-States)
