You can chose Turn-Based Mode in POE2 since patch 4.1. (Beta-Version)
Instead of all characters being free to take actions simultaneously, turn-based combat restricts the focus to one character at a time. Combat is taken in turn, with all combatants taking one turn each round. The combatants are queued based on their Recovery Time - now called "Initiative". Lower Initiative means acting earlier in the round, higher means acting later. Characters can only act (i.e. move, attack, and cast spells and abilities) during their turns. Once a character has taken their turn, the next character in the queue gets to move, and so on.
While the update doesn't change a lot of the actual health and damage mechanics of the game, it does change the effect of durations and over-time effects, converting them to last a number of rounds. This includes the regular tick, pulse and over-time effects, but also spell cast times and reloading.
The following is the comparison shown to the player starting a new game:
Real-Time with Pause Mode
In Turn-Based Mode, there are three type of actions. A free action does not use a character's action point. A character can perform a number of free actions on their turn in addition to Standard or Cast action.
Standard actions are the most common type of action. A standard action uses the character's action point for the turn and happens right away.
A cast action uses the character's action point. The character will immediately begin casting the ability. They will finish casting the ability later in the same round.
In turn-based mode, Initiative is used to determine who will act first in the turn order. Characters with lower initiative values will act before other characters and will complete spell casts faster.
Changes in Weapons/Armor/Accessories and other Equipment
Many equipment with time-related effects now scaled to rounds accordingly, see Accessories for more information. Weapons in Turn-based mode does not have a Rec Time, instead, they have a initiative themselves. Heavier weapons like crossbow usually have high initiative, causing your character to act after other people. (But they can still attack once every round.)
Note: Some Pets all also have different effect in turn based mode, like Abraham will reduce initiative, instead of reduce recovery time.