SELECT 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  feature_id = 30 
  AND product_id = 616 
  AND lang_code = 'en'

Query time 0.00055

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_features_values ref PRIMARY,fl,lang_code,product_id,fpl,product_id_idx,feature_id_idx,feature_value_product_idx,product_feature_idx PRIMARY 6 const,const 3 Using where; Using index

Result

variant_id
5933