Concurrent database connections using threads and mysqlplus. The following scenario will complete in roughly 3 seconds when db host is local, takes considerably longer on remote connections due to what appears to be blacking after 6 or 7 concurrent connections.

local-processlist

Id	User	Host		db	Command	Time	State		Info
1 	root	localhost:58150	NULL	Query	1131598	NULL		show processlist
2	root	localhost:58151	NULL	Sleep	7			NULL
1452	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1453	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1454	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1455	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1456	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1457	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1458	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1459	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1460	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1461	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1462	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1463	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1464	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1465	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1466	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1467	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1468	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1469	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1470	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1471	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1472	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1473	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1474	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1475	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1476	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1477	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1478	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1479	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1480	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1481	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1482	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1483	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1484	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1485	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1486	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1487	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1488	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1489	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1490	root	localhost	NULL	Query	1	User sleep	select sleep(3)
1491	root	localhost	NULL	Query	1	User sleep	select sleep(3)

mysql-concurrency.rb

gem 'activerecord', '=3.0.5'

require 'active_record'
require 'mysqlplus'
class Mysql; alias :query :async_query; end

CONNECTIONS = 40
THREADS = 40

ActiveRecord::Base.establish_connection(
  adapter:  'mysql',
  host:     'localhost',
  username: 'root',
  pool:     CONNECTIONS 
)

threads = []

THREADS.times do |n|
  threads << Thread.new do
    ActiveRecord::Base.connection_pool.with_connection do |connection|
      result = connection.execute('select sleep(3)')
    end
  end
end

threads.each {|t| t.join}

remote-processlist

Id	User	Host		db	Command	Time	State		Info
5  	root	localhost:54499	NULL	Sleep	7			NULL
7	root	remote:57616	root	Query	0	NULL		show processlist
8	root	remote:57617	NULL	Sleep	43			NULL
191	root	remote:59084	root	Sleep	10			NULL
192	root	remote:59085	root	Sleep	9			NULL
193	root	remote:59086	root	Sleep	8			NULL
194	root	remote:59087	root	Sleep	7			NULL
195	root	remote:59088	root	Sleep	6			NULL
196	root	remote:59089	root	Sleep	5			NULL
197	root	remote:59090	root	Sleep	4			NULL
198	root	remote:59091	root	Sleep	4			NULL
199	root	remote:59092	root	Query	2	User sleep	select sleep(3)
200	root	remote:59093	root	Query	1	User sleep	select sleep(3)
201	root	remote:59094	root	Sleep	1			NULL