Hi there,
I have the unique shield Rahyeh’s Light which provides 50% less fire spell duration.
My understanding from this wording is that it should lower the duration of Infernal Shade. But quick testing ingame seems to show no difference even though numerically it seems like it’d be very obvious.
For full disclosure, I have Infernal Shade with 5/5 Wildfires and Combustion. Obviously the idea is to lower the duration significantly in order to rely on explosions for damage. But the shield doesn’t seem to actually lower the duration any further.