https://www.pcgamer.com/rainbow-six-siege-ama-2018/

During an AMA today on the game’s subreddit with members of the balancing team, Rainbow Six Siege developers hinted at changes to the speed and armor dynamic of the game coming soon. Currently the mobility and durability of Siege's 36 operators are rated between one and three. Bulkier operators like Rook move more slowly and have larger hitboxes, but in theory make up for their slowness with greater damage resistance. However, in practice, fast-movers with smaller hitboxes tend to win gunfights if they make small, quick peeks.

User PhD_Bagel asked a question on behalf of the Doc and Rook mains of the world: “[Are] there any plans to balance/change speed and armor? Currently, it seems that 3 speed is far superior to 3 armor.”