This is in reference to Does casting a long spell end a maintained Concentration spell?
Casting spells that take more than an action require concentration, so this means you have to drop your concentration spell to cast them.
Slow can make spells take more than one action, does this mean that if my wizard has Detect Magic up, is affected by Slow and casts Magic Missile (which now takes longer than one action to complete); do I pick if the Magic Missile goes off or the Detect Magic is dropped?
Casting Time is an attribute of a spell, just as Speed is an attribute of a character. The Slow spell reads:
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.
If the Slow spell was meant to alter the Casting Time, the authors would have used the much simpler construction, "On an 11 or higher, the spell has a Casting Time of 2 Actions."
The rules mean exactly what they say: the Casting Time remains 1 action, but the spell does not take effect until the next round, and the creature must use a second action to complete (not "cast") the spell.
Note that the Slow spell affects up to six creatures in a 40' cube at the rather hefty range of 120'. To be able to potentially bring down the concentration spells (or otherwise prevent further castings) of so many enemies at such range and area would be pretty impressive for a mere third level spell. Even with this less generous interpretation, Slow roughly halves the casting power of a typical spellcaster and similarly halves the firepower of most other classes. As written, Slow is an outstanding spell and hardly needs any further buffing to be worth a page in every wizards' spellbook.
Longer Casting Times doesn't make any exceptions for the cause of the longer casting time; if it takes more than a single action or a reaction, it requires concentration.
Concentration says you lose concentration on a spell if you cast another spell that requires concentration. You can't concentrate on two spells at once.
The only choice is starting to cast another spell, or not. If you start another spell that requires concentration, concentration on the first spell is lost. What causes the second spell to require concentration is not a factor.
Slow has a 50% chance to turn a single action spell into a multiple action spell. As the character doesn't know exactly how long the spell will take, if they risk it, they risk suffering the consequences. The player can decide if the character wants to take the chance.
