Sandbox

SELECT 
  pf.feature_id, 
  pf.feature_style, 
  pf.position, 
  pf.purpose, 
  pf.display_on_catalog, 
  pfd.description, 
  pfd.internal_name, 
  pfd.prefix, 
  pfd.suffix, 
  (
    CASE WHEN pf.purpose = 'group_catalog_item' THEN 0 ELSE 1 END
  ) AS purpose_position 
FROM 
  cscart_product_features AS pf 
  INNER JOIN cscart_product_features_descriptions AS pfd ON pf.feature_id = pfd.feature_id 
  AND pfd.lang_code = 'en' 
WHERE 
  pf.feature_id IN (548, 549) 
ORDER BY 
  purpose_position ASC, 
  pf.position ASC, 
  pf.feature_id ASC

Query time 0.00025

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	pf	range	PRIMARY	PRIMARY	3		2	Using index condition; Using filesort
1	SIMPLE	pfd	eq_ref	PRIMARY	PRIMARY	9	cscart_dev_4_16.pf.feature_id,const	1	Using index condition

Result

feature_id	feature_style	position	purpose	display_on_catalog	description	internal_name	prefix	suffix	purpose_position
549	dropdown_images	10	group_catalog_item	Y	Color	Color			0
548	dropdown_labels	20	group_variation_catalog_item	Y	Size	Size			1