Ran into a weird issue when playing around with the new Hillman Imp. Whenever the car would flip/roll and cut away to the flip/roll cinematic camera the frame rate would drop/stutter. This wasn’t tied to a specific location - I rolled it in a handful of places testing it out. And when I switched to a different car the frame rate problem didn’t happen.
This issue should be resolved by the Series 6 content update being released today or tomorrow:
FH4 Release Notes: February 12th, 2019
https://support.forzamotorsport.net/hc/en-us/articles/360017713494-FH4-Release-Notes-February-12th-2019