In the xbox series s one issue that has persisted since launch is that in the 120 hz mode when you ult as widowmaker or use the alternate form as bastion the performance drops consistently to 80 fps with seemingly bad frame pacing, it only hapens from their perspective.
If my teammate widow ults my performance is fine, it only happens when im the widow.
My suspition is it might have something to do with the screen effect when you ult, maybe a fill rate problem with some transparency.