slow query on myisam table

[Jan Kowalski]

I have server with many wordpress instalations and there is problem with this query:
SELECT option_name, option_value FROM wp_11_options WHERE autoload = 'yes'

CREATE TABLE `wp_options` (
`option_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`option_name` varchar(64) NOT NULL DEFAULT '',
`option_value` longtext NOT NULL,
`autoload` varchar(20) NOT NULL DEFAULT 'yes',
PRIMARY KEY (`option_id`),
UNIQUE KEY `option_name` (`option_name`)
)

There is 689 wp_*_options tables on server and on average 500 rows per table. There is no need to index autoload column because there is 'yes' value in about 80% of rows

this is output of mq-query-digest:
# Query 1: 0.12 QPS, 0.22x concurrency, ID 0xFEC9B62C781B75A2 at byte 380885
# This item is included in the report because it matches --limit.
# Scores: Apdex = 0.51 [1.0], V/M = 0.36
# Query_time sparkline: | ^ |
# Time range: 2014-03-04 11:41:58 to 13:01:08
# Attribute pct total min max avg 95% stddev median
# ============ === ======= ======= ======= ======= ======= ======= =======
# Count 35 591
# Exec time 28 1042s 1s 9s 2s 3s 792ms 1s
# Lock time 54 10s 31us 724ms 18ms 87ms 78ms 66us
# Rows sent 43 141.30k 153 289 244.82 271.23 24.34 234.30
# Rows examine 0 237.80k 225 600 412.02 563.87 89.39 381.65
# Query size 13 42.61k 73 75 73.83 72.65 0 72.65
# String:
# Databases zaplecze2_... (219/37%), zaplecze5_... (99/16%)... 3 more
# Hosts portal-portali.pl
# Users zaplecze2_... (219/37%), zaplecze5_... (99/16%)... 3 more
# Query_time distribution
# 1us
# 10us
# 100us
# 1ms
# 10ms
# 100ms
# 1s ################################################################
# 10s+
# Tables
# SHOW TABLE STATUS FROM `zaplecze2_pizpl` LIKE 'wp_11_options'\G
# SHOW CREATE TABLE `zaplecze2_pizpl`.`wp_11_options`\G
# EXPLAIN /*!50100 PARTITIONS*/
SELECT option_name, option_value FROM wp_11_options WHERE autoload = 'yes'\G


Is there any way to optimise this query?