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 (78, 57, 81) 
  AND pfv.product_id IN (5476, 104, 390) 
  AND pfv.lang_code = 'en'

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "pfv",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "variant_id",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["feature_id", "product_id"],
      "rows": 10,
      "filtered": 90,
      "attached_condition": "pfv.feature_id in (78,57,81) and pfv.product_id in (5476,104,390) and pfv.lang_code = 'en'",
      "using_index": true
    },
    "table": {
      "table_name": "fv",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "feature_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "ref": ["herbaria_cscart_4_18.pfv.variant_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "fv.feature_id = pfv.feature_id"
    },
    "table": {
      "table_name": "fvd",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "ref": ["herbaria_cscart_4_18.pfv.variant_id", "const"],
      "rows": 1,
      "filtered": 100,
      "index_condition": "fvd.lang_code = 'en'"
    }
  }
}