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 (
    20852, 20867, 20835, 21923, 20635, 19817, 
    21055, 20849, 21288, 21072, 21029, 
    21015, 20051, 20636, 20721, 20868, 
    19685, 21049, 21915, 23715, 20512
  ) 
  AND pfv.product_id IN (
    20591, 2912, 3006, 1043, 3449, 6044, 
    2460, 20588, 2018, 2443, 2497, 2511, 
    5416, 3448, 3196, 2911, 14985, 2466, 
    1056, 36492, 3712
  ) 
  AND pfv.lang_code = 'en'

Query time 0.06481

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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": 481,
          "filtered": 0.030383999,
          "attached_condition": "pfv.feature_id in (20852,20867,20835,21923,20635,19817,21055,20849,21288,21072,21029,21015,20051,20636,20721,20868,19685,21049,21915,23715,20512) and pfv.product_id in (20591,2912,3006,1043,3449,6044,2460,20588,2018,2443,2497,2511,5416,3448,3196,2911,14985,2466,1056,36492,3712) 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": ["dev_db.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": ["dev_db.pfv.variant_id", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "fvd.lang_code = 'en'"
        }
      }
    ]
  }
}