WCF service Response Slow

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WCF

WPF

i'm using WCF service for the first time

i need to execute database queries from server (not from client machine).
Database Server SQL 2005


so i have used WCF service which execute database queries on server and reruns the result.

Service is hosted in IIS 7


initially it works fine but sometimes its response gets worst.

if i use Self Hosted WCF service the Database Queries Will Execute from client Machine only


How i can improved performance of my application using WCF service or should i use web service ?
or any other way.
please help..