That's a very good point. You and I actually play her enough to be effective.
There are numerous factors that could have impacted her win rate and more players would make it lower drastically.
I love Zyra and would love to see her get stronger. I just don't see the projectile speed being an issue. It hasn't been for me anyways.
I honestly don't know what a healthy win rate for a champion is at this point. Riot is making so many balance changes (as they should) during preseason its hard to tell. I think Zyra's biggest drawback is her lack of mobility. If you miss your E you can pretty much say goodbye. The meta champions are in your face right now deal with it type champions. Zyra want's you to stay away and play in the garden.