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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 (549, 548) 
  AND pfv.product_id IN (1, 341, 346, 351) 
  AND pfv.lang_code = 'en'

Query time 0.00073

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		9	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	1	1194	2	Large
549	1	1198	1	Green
548	341	1194	2	Large
549	341	1199	2	Blue
548	346	1194	2	Large
549	346	1200	3	Black
548	351	1194	2	Large
549	351	1201	4	White