What happens to a spell cast by a target affected by the delayed effect of the Slow spell when he saves?

Question

Part of the description of the Slow spell says:

If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.

The affected spellcaster casts a spell and is hindered by the Slow spell so that their spell does not go off until their next turn, but they save against the Slow spell at the end of their turn.

Answer 1

You get to roll save for slow at the end of turn. You don't get to do anything else on that turn and your spell is still half-complete. So, at the beginning of your next turn, you have two options - drop it or finish it with your action.

Compare it with movement under effects of slow. To get to your enemy - you need to go 30 feet. You move half your speed - 15 feet and your movement is over. You save successfully against slow at the end of turn, but enemy is still 15 feet away from you. And on your next turn - you need to spend your movement to cover those 15 feet.

Same with casting spells - you've rolled your high save at the end of turn, but you still have that half of spell to finish and you need to spend action on it. And, since rules don't allow to 'fraction' casting normally (unlike movement) you can't use remaining bit of action to do something else.