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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 (278) 
  AND pfv.lang_code = 'en'

Query time 0.00063

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	fv	range	PRIMARY,feature_id	feature_id	3		7	Using index condition
1	SIMPLE	pfv	eq_ref	PRIMARY,fl,variant_id,lang_code,product_id,fpl,idx_product_feature_variant_id	PRIMARY	15	cscart_dev_4_16.fv.feature_id,const,cscart_dev_4_16.fv.variant_id,const	1	Using where; Using index
1	SIMPLE	fvd	eq_ref	PRIMARY	PRIMARY	9	cscart_dev_4_16.fv.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