ROLAP Cube is a datastructure that is used for faster analysis of data.ROLAP is slow at analysis but large at capacity(Multibillion rows can b stored) u can use good indexes to improve its performance

ROLAP Cubes: stands for Relational OLAP. In ROLAP cubes a 
copy of data from the fact table is not made and the data 
aggregates are stored in tables in the source relational 
database. A ROLAP cube is best when there is limited space 
on the Analysis Server and query performance is not very 
important. ROLAP local cubes contain the dimensions and 
cube definitions but aggregates are calculated when they 
are needed. ROLAP cubes require less storage space than 
MOLAP and HOLAP cubes.