Mysql – Why using where in indexed field is slow

we have a big table. the table structure is :

CREATE TABLE `visit_logs` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `user_id` int(10) NOT NULL,
  `ref_id` int(10) NOT NULL DEFAULT '0',
  `link_id` int(12) NOT NULL,
  `date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `day_of_week` tinyint(1) NOT NULL,
  `jalali_month` tinyint(2) NOT NULL ,
  `value` int(9) NOT NULL ,
  `ref_share` float(4,2) NOT NULL DEFAULT '0.00',
  `balance` int(9) NOT NULL ,
  `ads_id` int(12) NOT NULL,
  `ip` varchar(15) NOT NULL,
  `hash` varchar(32) NOT NULL,
  `referer` text,
  `shw` text,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`),
  KEY `jalali_month` (`jalali_month`),
  KEY `value` (`value`),
  KEY `ads_id` (`ads_id`),
  KEY `ip` (`ip`),
  KEY `hash` (`hash`),
  KEY `link_id` (`link_id`),
  KEY `day_of_week` (`day_of_week`),
  KEY `ref_share` (`ref_share`),
  KEY `ref_id` (`ref_id`),
  KEY `date` (`date`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8;

these are some sample queries :

mysql> SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` where id>13197856;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 10443393 | 200940970 |
+----------+-----------+
1 row in set (6.02 sec)

mysql> SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` where `date`  > NOW() - INTERVAL 90 day;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 10443354 | 200940430 |
+----------+-----------+
1 row in set (9.24 sec)

mysql> SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` ;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 23641291 | 596178719 |
+----------+-----------+
1 row in set (3.32 sec)

as you can see , selecting all rows is faster than selecting some row filtered by id , also filter by id is faster than filter by timestamp . id and timestamp fields are indexed !

Mysql version is 5.1 .
my.conf :

thread_concurrency = 8
query_cache_size = 1G
thread_cache_size = 8
myisam_sort_buffer_size = 1G
read_rnd_buffer_size = 256M
read_buffer_size = 512M
sort_buffer_size = 512M
table_open_cache = 512
max_allowed_packet = 20M
key_buffer_size = 1G
#log = /var/log/mysqlq.log
wait_timeout=120
connect_timeout=50
#max_execution_time=60000
tmp_table_size=2G
max_heap_table_size=2G

Explain :

explain SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` where id>13197856;
+----+-------------+------------+------+---------------+------+---------+------+----------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows     | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+----------+-------------+
|  1 | SIMPLE      | visit_logs | ALL  | PRIMARY       | NULL | NULL    | NULL | 12978097 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+----------+-------------+
1 row in set (0.00 sec)
mysql> explain SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` force index (PRIMARY) where id>13197856;
+----+-------------+------------+-------+---------------+---------+---------+------+----------+-------------+
| id | select_type | table      | type  | possible_keys | key     | key_len | ref  | rows     | Extra       |
+----+-------------+------------+-------+---------------+---------+---------+------+----------+-------------+
|  1 | SIMPLE      | visit_logs | range | PRIMARY       | PRIMARY | 4       | NULL | 10604250 | Using where |
+----+-------------+------------+-------+---------------+---------+---------+------+----------+-------------+
1 row in set (0.00 sec)

mysql>  SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` force index (PRIMARY) where id>13197856;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 10534567 | 202721650 |
+----------+-----------+
1 row in set (10.73 sec)

mysql>  SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` force index (PRIMARY) where id>13197856;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 10534578 | 202721830 |
+----------+-----------+
1 row in set (10.75 sec)

mysql>  SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs`  where id>13197856;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 10534588 | 202722030 |
+----------+-----------+
1 row in set (3.50 sec)

mysql>  SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs`  where id>13197856;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 10534592 | 202722070 |
+----------+-----------+
1 row in set (3.53 sec)

mysql>  SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` ;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 12978727 | 262590120 |
+----------+-----------+
1 row in set (1.66 sec)

mysql>  SELECT count(*),SUM( value ) AS `revenue` FROM `visit_logs` ;
+----------+-----------+
| count(*) | revenue   |
+----------+-----------+
| 12978730 | 262590180 |
+----------+-----------+
1 row in set (1.65 sec)