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SELECT 
  pfv.feature_id, 
  pfv.product_id, 
  pfv.variant_id, 
  fv.position, 
  fvd.variant 
FROM 
  cscart_product_features_values AS pfv 
  INNER JOIN cscart_product_feature_variants AS fv ON pfv.feature_id = fv.feature_id 
  AND pfv.variant_id = fv.variant_id 
  INNER JOIN cscart_product_feature_variant_descriptions AS fvd ON pfv.variant_id = fvd.variant_id 
  AND fvd.lang_code = 'en' 
WHERE 
  pfv.feature_id IN (548, 549) 
  AND pfv.product_id IN (280, 281, 282, 283, 284, 278, 279) 
  AND pfv.lang_code = 'en'

Query time 0.00137

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	pfv	range	PRIMARY,fl,variant_id,lang_code,product_id,fpl,idx_product_feature_variant_id	idx_product_feature_variant_id	12		17	Using where; Using index
1	SIMPLE	fv	eq_ref	PRIMARY,feature_id	PRIMARY	3	cscart_dev_4_16.pfv.variant_id	1	Using where
1	SIMPLE	fvd	eq_ref	PRIMARY	PRIMARY	9	cscart_dev_4_16.pfv.variant_id,const	1	Using index condition

Result

feature_id	product_id	variant_id	position	variant
548	278	1195	3	Medium
549	278	1201	4	White
548	279	1194	2	Large
549	279	1201	4	White
548	280	1195	3	Medium
549	280	1200	3	Black
548	281	1194	2	Large
549	281	1200	3	Black
548	282	1195	3	Medium
549	282	1199	2	Blue
548	283	1194	2	Large
549	283	1199	2	Blue
548	284	1193	1	Small
549	284	1199	2	Blue