Hi pokipoka,
we have observed that in the vast majority of cases the DNA tiles are used at the beginning of the turn, before accepting or refusing the hand. You have plenty of time to choose.
Once the main action is done, the rule says that it is still possible to use the DNA tiles, but in order not to break the rhythm of the game (to avoid unnecessary waiting time and forgetting to click on a "validate" for example), a timer is triggered, as it is done in many other games.
That said, a slightly experienced player who specifically wants to use a tile after his action can click on the "I want to use a tile" button and then he has plenty of time to think about it