Welcome to the MongoDB Community @Huang_Huang !
Amazon DocumentDB is an independent emulation of a subset of features for the associated MongoDB server version they claim compatibility with.
The server implementations do not have any code in common, so it is not surprising if there are differences (sometimes significant) in behaviour and feature support.
If you are trying to understand performance issues for DocumentDB, I recommend asking on Stack Overflow or an AWS product community: Newest 'aws-documentdb' Questions - Stack Overflow.
Regards,
Stennie