RDBMSs use a table-based normalization approach to data, and that's a limited model. Certain data structures cannot be represented without tampering with the data, programs, or both. They allow versioning or activities like: Create, Read, Update and Delete. For databases, updates should never be allowed, because they destroy information. Rather, when data changes, the database should just add another record and note duly the previous value for that record. Performance falls off as RDBMSs normalize data. The reason: Normalization requires more tables, table joins, keys and indexes and thus more internal database operations for implement queries. Pretty soon, the database starts to grow into the terabytes, and that's when things slow down.
via NoSQL databases