Why is this table structure so slow to CRUD?

Question

I have a table that I am using for queuing emails. Unfortunately, adding to or reading from this table becomes horrendously slow after adding about 50 records.

-- Table "mail_queue" DDL
CREATE TABLE `mail_queue` (
  `id` bigint(20) NOT NULL DEFAULT '0',
  `create_time` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `time_to_send` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `sent_time` datetime DEFAULT NULL,
  `id_user` bigint(20) NOT NULL DEFAULT '0',
  `ip` varchar(20) NOT NULL DEFAULT 'unknown',
  `sender` varchar(50) NOT NULL DEFAULT '',
  `recipient` text NOT NULL,
  `headers` text NOT NULL,
  `body` mediumblob NOT NULL,
  `try_sent` tinyint(4) NOT NULL DEFAULT '0',
  `delete_after_send` tinyint(1) NOT NULL DEFAULT '1',
  PRIMARY KEY (`id`),
  KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

I have tried using a text as well as a blob for the email body field, but no noticeable performance change, either way.

Thanks for any help.

Results of "SHOW VARIABLES LIKE 'innodb%';":

Variable_name   Value
innodb_adaptive_flushing    ON
innodb_adaptive_hash_index  ON
innodb_additional_mem_pool_size 8388608
innodb_autoextend_increment 8
innodb_autoinc_lock_mode    1
innodb_buffer_pool_instances    1
innodb_buffer_pool_size 134217728
innodb_change_buffering all
innodb_checksums    ON
innodb_commit_concurrency   0
innodb_concurrency_tickets  500
innodb_data_file_path   ibdata1:10M:autoextend
innodb_data_home_dir    
innodb_doublewrite  ON
innodb_fast_shutdown    1
innodb_file_format  Antelope
innodb_file_format_check    ON
innodb_file_format_max  Antelope
innodb_file_per_table   OFF
innodb_flush_log_at_trx_commit  1
innodb_flush_method 
innodb_force_load_corrupted OFF
innodb_force_recovery   0
innodb_io_capacity  200
innodb_large_prefix OFF
innodb_lock_wait_timeout    50
innodb_locks_unsafe_for_binlog  OFF
innodb_log_buffer_size  8388608
innodb_log_file_size    5242880
innodb_log_files_in_group   2
innodb_log_group_home_dir   ./
innodb_max_dirty_pages_pct  75
innodb_max_purge_lag    0
innodb_mirrored_log_groups  1
innodb_old_blocks_pct   37
innodb_old_blocks_time  0
innodb_open_files   300
innodb_purge_batch_size 20
innodb_purge_threads    0
innodb_random_read_ahead    OFF
innodb_read_ahead_threshold 56
innodb_read_io_threads  4
innodb_replication_delay    0
innodb_rollback_on_timeout  OFF
innodb_rollback_segments    128
innodb_spin_wait_delay  6
innodb_stats_method nulls_equal
innodb_stats_on_metadata    ON
innodb_stats_sample_pages   8
innodb_strict_mode  OFF
innodb_support_xa   ON
innodb_sync_spin_loops  30
innodb_table_locks  ON
innodb_thread_concurrency   0
innodb_thread_sleep_delay   10000
innodb_use_native_aio   ON
innodb_use_sys_malloc   ON
innodb_version  1.1.8
innodb_write_io_threads 4

Version: 5.5.22-cll