In addition to what afoken said, try adding indices to your table.
CREATE INDEX CT_Media_Relations_ix_SupplierID
ON CT_Media_Relations (SupplierID);
CREATE INDEX CT_Media_Relations_ix_SupplierPID
ON CT_Media_Relations (SupplierPID);
Indices will somewhat slow down write queries (INSERT, etc) but can have a huge improvement on read queries (SELECT, etc).
Any primary keys will already be indexed, but any other columns containing numbers, dates, and short strings which you frequently filter, sort, or group by should probably be indexed.