Simple RAW for 5e V: The Disadvantage Strikes Back

[Hairfish]

Q 410
Can a Gynosphinx (or another creature with a similar legendary action) use their Cast a Spell legendary action to bypass the normal casting time or action requirements of a spell on their list? For example, casting Legend Lore with one legendary action or casting Shield without using a reaction against an attack that would hit the Sphinx?

A 410
No. A legendary action may only be used at the end of another creature's turn and gives no additional benefits (such as reduced casting times) unless it specifically says they do.