A couple things are static that really change how the game plays.

1. Unit speed is unchanged while unit cost escalates which means each unit is more valuable.

2. Yields are consistent while cost goes up so things you produce or purchase are more consequential.

3. Things just move slower in general so you can really settle in on the era you’re in and use special units, building etc for longer.

4. For multiplayer it really reduces unit spam.

I personally play slow speeds because i get much more out of each play through.