Hi Andrey,
To answer your questions/points:
1) It's one of those situations where something that is a benefit on small, simple queries suddenly becomes a massive problem on complex queries. It's not a bug, but it's something MS really should deal with better I think.
2) I'm not quite clear from your explanation whether Q1 and Q2 are queries or steps within the same query - I guess you mean they are queries? If so, yes I think you have understood the same thing from Ehren's explanation that I have. As he says:
However, because buffering happens in memory and is not persisted on disk, buffering during the load of one query does not affect the load of another query.
Chris
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