The rules for readying spells says the following:
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.
I saw this argument on reddit today:
The rules say you cast the spell as normal, so the spell takes effect immediately. I have no idea what the energy stuff is about, but it says you cast the spell as normal.
It seems to me like the intention is for the spell to only take effect when the trigger occurs and the caster chooses to activate the spell. I don’t have any doubt that’s how it’s supposed to work.
However I think the reddit user has a point, the whole "energy" thing is a bit ill-defined. Is there a concrete text in the rules that proves that "holding back energy and releasing it" change the timing of a spell being cast?