Subject Foreign Key & Query Performance

Hello there,

I am having a doubt that if Foreign Key reduces Query Performance Or not.

My Environment is 

FirebirdCS2.6 :-

Item Table :- 10 Records, PK - Itemcode Varchar(5);

BillDetail Table :- 10 Million Records (FK Selectivity - 0.0076, PK Selectivity - 0.000000076)

I am having FK between BillDetail.Itemcode And Item.Itemcode with FK every query is slow. but as i remove the FK and added Index with Itemcode&BillDetail_PK the queries are running lightning fast. But I prefer FK to be in Database with high performance queries.

Kindly share you suggestions and Feedbacks on the same.