Chandra's Phoenix is not "too slow" if we are comparing it against Sulfuric Vortex.
Chandra's Phoenix:
- Deals two damage right away instead of waiting for the opponents upkeep.
- More resilient to removal compared to Vortex. Requires a swords to remove, which means one less swords for your Guides or Devils. Bolts and blockers won't stop it forever.
- Doesn't deal damage to you, making it better against other aggro decks looking to race you.
Both cards kill at about the same speed. The Phoenix is better against blind matches compared to Vortex. If you were considering main-decking Vortex, put that may birds in it's place and put the Vortexes in the side board.