For simple things, you can try
Engine.time_scale. Setting it to 1 means normal speed, to 0.1 means ten times slower, etc. For example
Engine.time_scale = 0.5
Makes the game run at half speed. However, i found that some collisions may show strange behaviours.. you should try and see if its enough for you.
BTW, according to reddit, this is what securas used in his github game off jam winner, Sealed Bite.