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SELECT 
  pfv.feature_id, 
  pfv.product_id, 
  pfv.variant_id, 
  gp.group_id 
FROM 
  cscart_product_features_values AS pfv 
  INNER JOIN cscart_product_variation_group_products AS gp ON pfv.product_id = gp.product_id 
  INNER JOIN cscart_product_variation_group_features AS gpf ON gpf.group_id = gp.group_id 
  AND gpf.feature_id = pfv.feature_id 
WHERE 
  pfv.lang_code = 'en' 
  AND gp.group_id IN (12)

Query time 0.00045

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	gpf	ref	PRIMARY,idx_group_id	idx_group_id	3	const	1
1	SIMPLE	gp	ref	PRIMARY,idx_group_id	idx_group_id	3	const	4
1	SIMPLE	pfv	ref	PRIMARY,fl,lang_code,product_id,fpl,idx_product_feature_variant_id	idx_product_feature_variant_id	12	cscart_dev_4_16.gp.product_id,cscart_dev_4_16.gpf.feature_id,const	1	Using where; Using index

Result

feature_id	product_id	variant_id	group_id
548	162	1193	12
548	290	1195	12
548	291	1197	12
548	292	1196	12