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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 (
    341, 346, 351, 286, 287, 288, 278, 280, 
    282
  ) 
  AND pfv.lang_code = 'en'

Query time 0.00095

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	fvd	eq_ref	PRIMARY	PRIMARY	9	cscart_dev_4_16.fv.variant_id,const	1	Using index condition
1	SIMPLE	pfv	ref	PRIMARY,fl,variant_id,lang_code,product_id,fpl,idx_product_feature_variant_id	fl	12	cscart_dev_4_16.fv.feature_id,const,cscart_dev_4_16.fv.variant_id	3	Using index condition; Using where

Result

feature_id	product_id	variant_id	position	variant
548	341	1194	2	Large
548	346	1194	2	Large
548	351	1194	2	Large
548	278	1195	3	Medium
548	280	1195	3	Medium
548	282	1195	3	Medium
549	282	1199	2	Blue
549	286	1199	2	Blue
549	341	1199	2	Blue
549	280	1200	3	Black
549	287	1200	3	Black
549	346	1200	3	Black
549	278	1201	4	White
549	288	1201	4	White
549	351	1201	4	White