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 = 'de' 
WHERE 
  pfv.feature_id IN (51, 40, 37) 
  AND pfv.product_id IN (813, 814, 196, 165) 
  AND pfv.lang_code = 'de'

Query time 0.00100

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.053699815,
    "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": "fl",
          "key_length": "9",
          "used_key_parts": ["feature_id", "lang_code"],
          "loops": 1,
          "rows": 35,
          "cost": 0.012572455,
          "filtered": 34.2857132,
          "attached_condition": "pfv.feature_id in (51,40,37) and pfv.product_id in (813,814,196,165) and pfv.lang_code = 'de'",
          "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": ["u428615623_ecartifygonje.pfv.variant_id"],
          "loops": 12,
          "rows": 1,
          "cost": 0.02056368,
          "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": ["u428615623_ecartifygonje.pfv.variant_id", "const"],
          "loops": 12,
          "rows": 1,
          "cost": 0.02056368,
          "filtered": 100,
          "attached_condition": "fvd.lang_code = 'de'"
        }
      }
    ]
  }
}