It appears that the very initial star that hits from right click is faster than that of the left click. Is this true?
This guy says he tested the projectile speeds and states the shurikens are thrown at 50 m/s - which implies both shuriken throws are done at the same speed.
More information obtained. Someone made a google sheets detailing many aspects of the game, one of them being projectile speeds.
He lists Genji's shurikens (so assuming both fires tested) at 47 m/s. Assuming both tested and they are the same because he lists alt fires for other heroes such as roadhog.
This is the original thread it came from, but the sheet was trashed, so the linked one is a recovered version.