Carol doesn't go FTL in combat. Thanos was able to react to her.
Who says she has to go FTL in combat?
Outlier for Thanos
Jean reacted to Quicksilver, who is MHS+, and Storm's lightning.
Carol doesn't abuse her speed in combat, especially not on land. And Jean's powers work at the speed of thought and require no travel time since they aren't projectiles.
Everyone here flies/levitates, right? So battle looks like an aerial one. What about when she used her speed fighting the Kree ships and stuff?
Speaking of projectiles, wasn't her reaction against QS moving stuff out of his way to trip him up? Not sure how fast Storm's lightning considering how it fared relative to what Hank shot, etc. Should be good if assumed to be fast as real lightning.