it is depends on
Data base design.
Hardware Sizing,
Application T- SQL codes- Indexes,Stored 
Procedues,constraints
no of clients
no users
Type of data and Quantity base storage capacity on daily 
basis

first we have to check indexes are there or not,
check whether the statistics are updated or not,
check fragmentation is there or not,
check availability of joins,
check availability of aggregatefnctions,
may be networking problem,
may be hardware problem

then we tunne according to these problems