UngeheuerLich
Legend
Yes, about the same amount of time. And since the caster started first, they finish first. The attack lands after the spell is finished.
A caster can manage two spells a round plus movement with 1 action cantrips and a bonus action spell. Combat magic in 5e is fast.
There is no "Interrupt Casting" action in 5e, nor is casting a 1 action spell subject to Concentration checks for suffering damage. An unprotected Wizard swimming through acid isn't required to make a Concentration check to cast a 1 action spell.
I would always go with the interpretation that is more fun. I would not want it to be an action that drags combat. But in a situation where a readied crossbow is pointed at the wizard and it is: "hold still or I shoot", then it would be appropriate to have the bolt attack resolved first.
On the other hand maybe it would be better to just start combat and rolling initiative, maybe with advantage on the initiative check for the crossbow shooter. Maybe I´d allow the wizard a bluff check to negate the advantage or allow for a surprise round. On a failed bluff check I might even give the wizard disadvantage on the dex check. Remember that initiative is only a dex check on which the DM can give advantage or disadvantage depending on the circumstances.