Yeah, this scene right?
He was slower than the bullet in that scene. In fact, we know that Quicksilver isn't that much faster than bullets cause he died from bullets from a minigun(admittedly should be faster than handgun bullets).
And another thing, Thanos fights at a speed manageable for characters like ironman, cap and Wanda, whom have only human reaction speed.
So it makes no sense for MCU Thanos to be even bullet speed.