Delay between the moment you press the key to cast a spell and the moment you can cast it on the map

Don't know if it come from me or the client but, in several games, I had the case where I want to cast a spell (any kind of spell, target or skillshot doesn't matter) and the delay between the moment I press the key and the moment the spell play on the screen (or the arrow/circle to show the range appears) is 1 or more second after.

At the begining I thought it was connection lag but every other thing like ennemy spell, champion path change, etc respond as usual.