Optimize SQL query - one hundred twenty-nine million records

Question

I have a table (UserDeptMapping) which conatins 129000000 (one hundred twenty-nine millions records).

 

 Department details table which has 500 entries.

 

ID

DeptId

WorldLocationId

RegionId

DistrictId

 

 

 Table UserDeptMapping which contains below data like

 

 

Id	UsertId	MappingId
1	1	1
2	1	2
3	1	3
4	2	1

 

Suppose execute below query against Department Id 1 like

Select * from UserDeptMapping Where MappingId = 2

 Output comes with total 300000 rows which is taking 30 seconds. 

How can I optimiase the query., so it will reduce the time.