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 = 'ja' 
WHERE 
  pfv.feature_id IN (7) 
  AND pfv.product_id IN (127) 
  AND pfv.lang_code = 'ja'

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "pfv",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "variant_id",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "idx_product_feature_variant_id",
      "key_length": "12",
      "used_key_parts": ["product_id", "feature_id", "lang_code"],
      "ref": ["const", "const", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "pfv.lang_code = 'ja'",
      "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": ["cscart_purchase_fl_com.pfv.variant_id"],
      "rowid_filter": {
        "range": {
          "key": "feature_id",
          "used_key_parts": ["feature_id"]
        },
        "rows": 7,
        "selectivity_pct": 4.487179487
      },
      "rows": 1,
      "filtered": 100,
      "attached_condition": "fv.feature_id = 7"
    },
    "table": {
      "table_name": "fvd",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "ref": ["cscart_purchase_fl_com.pfv.variant_id", "const"],
      "rows": 1,
      "filtered": 100,
      "index_condition": "fvd.lang_code = 'ja'"
    }
  }
}