The speed of the projectile is affected by attack speed mods and buffs.
The base attack speed of the Zaw has no influence on the projectile speed of Exodia Contagion.
Bug: For the explosion to count as a heavy attack, you need to be in the heavy attack animation when the explosion hits enemies. Which means you have to be close enough so the projectile hits before the animation ends. OR you can use a normal attack, and right before the projectile explodes use a heavy attack to ground slam for example.
Source: https://warframe.fandom.com/wiki/Exodia_Contagion?so=search
Heavy attack wind up times: https://warframe.fandom.com/wiki/Melee?so=search#Heavy_Attack