So if I have the Mage slayer feat and 5 feet away from a caster and he casts a spell, does my reaction attack come before, simultaneous with or after the spell effects?

My initial intuition is the reaction attack occurs simultaneously with the casting and prior to the effect, similar to counterspell which has the same trigger and happens before the spells effect takes place.

Example situations:

1. If he casts misty step do I get to attack him as he teleports away

2. If he casts hold person on me do I have to save against being paralyzed before I attack him or do I attack him and then save and if I attack him and hit him does he have to make a concentration check even though the spell is not in effect yet?

3. If he casts green-flame blade on me and my reaction attack kills him does he still get to roll his attack and damage?

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