Casting Time is not affected by Slow
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.