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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 (10)

Query time 0.00045

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	gp	ref	PRIMARY,idx_group_id	idx_group_id	3	const	7
1	SIMPLE	gpf	index	PRIMARY,idx_group_id	PRIMARY	6		6	Using where; Using index; Using join buffer (flat, BNL join)
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	278	1195	10
548	279	1194	10
548	280	1195	10
548	281	1194	10
548	282	1195	10
548	284	1193	10
548	283	1194	10
549	278	1201	10
549	279	1201	10
549	280	1200	10
549	281	1200	10
549	282	1199	10
549	284	1199	10
549	283	1199	10