It was so easy to be on fire in early days, Resurrect 1 person granting 100 fire point. And Mass Rez Mercy got buffs after buffs (Being able to move while casting Resurrect, 50hps to 60hps, self heal kicking after 3s to 1s, invulnerability while casting Resurrect, etc) As she got buffs, various fire point has been reduced to match on fire time.

And then, Valkyrie rework doubled player resurrected stat, increased offensive/defensive assists, increased healing, increased damage amplified, decreased death. To match on fire time, her various fire point has been drastically reduced. But then as Valkyrie Mercy got nerfs after nerfs, fire point has not been adjusted to match on fire time, leading Mercy to where she is, having lowest on fire time.