Increasing per_page scales poorly due to WP_REST_Posts_Controller::get_item_schema calls #2558

without schema caching:
per_page incl  self  called (overall time to run 5x http requests as reported by curl)
10       24.74 6.67  278    (3.577)
20       31.15 8.65  528    (5.701)
30       33.23 9.51  778    (7.199)
40       34.70 10.61 1028   (8.671)
100      37.85 10.80 2528   (18.925)

with schema caching:
per_page incl self called (overall time to run 5x http requests as reported by curl)
10       2.57 0.22 278    (2.899)
20       1.39 0.18 528    (3.946)
30       0.87 0.13 778    (4.887)
40       0.17 0.10 1028   (6.239)
100      0.28 0.04 2538   (12.072)

  private $schema;
  public function get_item_schema() {
    if ($this->schema) {
      return $this->schema;
    }

    $this->schema = parent::get_item_schema();
    return $this->schema;
  }