The main reason NoSQL dbms have become so common is the need of storing unstructured data where a Relational database instead would require carefully designing a database schema and fitting data into it.
What I don't understand is whether they are generally more performant than traditional relational databases and why. This of course under loads of data which both RDBMS and NoSQL can manage (like say a simple row with id,value,date).
To be more specific several threads on StackOverflow answering to questions like "What database can handle Millions rows" like SO question and SO question 2 always end up suggesting NoSQL databases. But apart from the specific structure of data I can't see why a NoSQL should be more performant than a traditional RDBMS.
Suggesting a RDBMS which can deal with millions of rows of structured data would also help.