There's no notable effects when time is burning. It acts exactly the same as when it's not burning to anyone that isnt specifically trying to use time magic to manipulate it.
Ignia set the city on fire with an attack. Mercphobia tried to put out the flames, but couldn't. Natsu ate the leftovers, and it gave him enough power to beat up Mercphobia. No matter how you spin it, the flames from one of Ignia's attacks gave Natsu enough power to beat Mercphobia. Like I said, Ignia's flames aren't lacking in raw power. They provide Natsu the strength to stomp Mercphobia. Ignia himself blasted a hole in Selene. Even if it has the hax to burn water, which doesnt help to burn Natsu anyways, that's still on top of having the raw power to blast holes in other god dragons.
Whether Zeref was crazy or not, he still hit Natsu several times. Natsu was literally covered in Zeref's magic, and wasnt close to dying. Even after Zeref got completely serious, he still couldnt kill Natsu with his curse. Wolfen seed was only killing Aldoron because he thinks Aldoron precious. Even then, the curse was travelling so slow that Natsu could outrun it. DF Natsu's attack did far more damage than Zeref's death wave. Ignia's flames can blast a hole through Selene, who is even stronger than Aldoron.