Drupal 4.7, old cache, 10,000 entries prefilled to create some data in the table:

execution 1
5000 iterations

    * Start Time Sat, 2006-06-24 20:39
    * Read time 6 sec (833.33333333333 per sec)
    * Write time 11 sec (454.54545454545 per sec)

execution 2
5000 iterations

    * Start Time Sat, 2006-06-24 20:39
    * Read time 6 sec (833.33333333333 per sec)
    * Write time 11 sec (454.54545454545 per sec)

execution 3
5000 iterations

    * Start Time Sat, 2006-06-24 20:40
    * Read time 5 sec (1000 per sec)
    * Write time 11 sec (454.54545454545 per sec)

HEAD running cache patch

execution 1
5000 iterations

    * Start Time Sat, 2006-06-24 20:40
    * Read time 6 sec (833.33333333333 per sec)
    * Write time 10 sec (500 per sec)

5000 iterations

    * Start Time Sat, 2006-06-24 20:41
    * Read time 7 sec (714.28571428571 per sec)
    * Write time 11 sec (454.54545454545 per sec)

5000 iterations

    * Start Time Sat, 2006-06-24 20:41
    * Read time 5 sec (1000 per sec)
    * Write time 11 sec (454.54545454545 per sec)

Code run:

<?php

function test_menu($may_cache) {

  if ($may_cache) {
    $items[] = array(
      'path' => 'test/reset',
      'callback' => 'test_reset',
      'title' => 'reset bench cache',
      'access' => true,
      'type' => MENU_NORMAL_ITEM
    );
    $items[] = array(
      'path' => 'test/bench',
      'callback' => 'test_bench',
      'title' => 'bench cache',
      'access' => true,
      'type' => MENU_NORMAL_ITEM
    );
  }
  return $items;
}

function test_reset() {
  cache_clear_all();
  foreach (range(0, 10000) as $i) {
    $j = md5($i);
    cache_set("temp:$j", $j . md5($i*$i) . md5($i*$i*$i));
  }
}

function test_bench($iter = 1000) {
  $start = time();

  // read iter entries
  foreach (range(0, $iter) as $i) {
    $j = md5($i);
    cache_get("temp:$j");
  }
  $post_read = time();
  // write iter entries
  foreach (range(0, $iter) as $i) {
    $j = md5($i);
    cache_set("temp:$j", $j . md5($i*$i) . md5($i*$i*$i));
  }
  $post_write = time();

  $post_del = time();
  $items[] = 'Start Time ' . format_date($start);
  $read_time = $post_read - $start;
  $items[] = 'Read time ' . format_interval($read_time) . ' (' . ($iter / $read_time) . ' per sec)';
  $write_time = $post_write - $post_read;
  $items[] = 'Write time ' . format_interval($write_time) . ' (' . ($iter / $write_time) . ' per sec)';

  return theme('item_list', $items, "$iter iterations");
}