Hi Sameer,
There will be plenty of documents on this in SCN. However, as a quick check, you can dig into the following items to see if any of them can help:
a) go to ST03N to analyze query performance with respect to DB time, OLAP time & Frontend Time.
b) if DB time is more, you can think of applying aggregates, compression, indexing, partitioning of Cubes
c) if OLAP time is more, try to increase its Cache
d) if front-end time is more, try to use less characteristics in rows which are initially displayed in the initial report.
e) if BW system supports BIA, include your Cube in BIA as this will increase report performance.
f) check if you have used more Nav Attr in reports
Thanks,
Joy.