I think when you look at Captain America 'stopping' Thanos' hit, He's not really holding him back, Thanos is more curious/amused at how hard Cap'n is fighting. He's not struggling against Captain America. He's sort of observing. But then again, looking at his face, he's maybe giving some effort there.. or angry about it. hard to tell.
But I think Thanos was giving his all against Hulk. Thanos didn't mess around with him.