Much like Magic: The Gathering, cards in Legends of Runeterra can have different speeds. For now, in Runeterra, these card speeds are pretty much limited to just spells, but that’s still enough to create a few relatively complex interactions.
There are a total of three different spell speeds in Runeterra: Slow, Fast, and Burst.
Slow spells are what you’d expect to find in any other card game; you play them when it’s your turn, and they have an effect. They can be interacted with if your opponent has a spell that can be played off turn to counter it, but otherwise, go off in any on-combat situation. Slow spells can only be played: out of combat”, as the game says. What this means is that once the attack or blocking phase has started, you can no longer play Slow spells.
Fast spells, on the other hand, can be played at any time, even when it’s not your turn, and even in combat. While they are much faster than Slow spells, they are still interactable. Your opponent can play another Fast or Burst spell to counter yours or use some other effect to negate potentially. For example, if you cast Blade’s Edge to deal 1 damage to one of their units, they may play Radiant Strike to boost their unit’s health to let it survive).
Finally, Burst spells may be played at any time and are completely non-interactable by your opponent; a Burst spell goes off no matter what; not even other Burst spells can be played to interact with their effects.
This leads to an interesting back and forth within the round, while not leading to too much tedium by going too deep into the dreaded “stack,” as currently, no true Counterspell type card exists; only counters via discrete interacting effects that the other player can plan around.