MySQL slow server in a Billing application i have on server DELL R220 with 8Gb Ram and 60GB HDD CPU 2.8Mhz

anyway, this server is used to host the Radius Manager one of the telecom billing servers build on FreeRadius and with CentOS 6.

we have only 200 users, meantime we have more than 2 million voucher generated in the system and stored in the database.

the issue that when any inquiry is made to the users table the response time is very long and i do not know why.

i'm not expert and would appreciate you help to address the issue.
Add an index to the users table. This should decrease the response time when the users table is queried. If you can share an example SQL statement used to query the users table, I can provide a more specific recommendation. 

The columns in parenthesis should be the columns used in the WHERE clause of your SQL query. 

CREATE INDEX index_name
ON table_name (column1, column2, ...);