Mysql sub query with MAX - mysql

Question

I'm trying to speed up a query which currently takes 1.2 seconds to run. The query is:
select
process.exchange,
process.market,
process.volume,
process.bid,
process.ask,
(select MAX(last) FROM a where a.exchange = process.exchange
AND a.market = process.market
AND a.created_date > NOW() - INTERVAL 5 MINUTE LIMIT 1) as Ask1,
Ask2,
((Ask2 / (select MAX(last) FROM a where a.exchange = process.exchange
AND a.market = process.market
AND a.created_date > NOW() - INTERVAL 5 MINUTE LIMIT 1 ))
* 100) - 100 as percentage
FROM process
WHERE process.exchange IN('BLAH','BLAH2')
ORDER BY percentage ASC
Table structures:
CREATE TABLE `a` (
`id` int(10) UNSIGNED NOT NULL,
`exchange` varchar(15) NOT NULL,
`market` varchar(15) NOT NULL,
`volume` double UNSIGNED NOT NULL,
`bid` double NOT NULL,
`ask` double UNSIGNED NOT NULL,
`last` double NOT NULL,
`created_date` datetime NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
ALTER TABLE `a`
ADD PRIMARY KEY (`id`),
ADD KEY `market` (`market`),
ADD KEY `exchange` (`exchange`),
ADD KEY `created_date` (`created_date`);
SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET time_zone = "+00:00";
CREATE TABLE `process` (
`id` int(11) NOT NULL,
`exchange` varchar(20) NOT NULL,
`market` varchar(10) NOT NULL,
`volume` double NOT NULL,
`bid` double NOT NULL,
`ask` double NOT NULL,
`Ask2` double NOT NULL,
`created_date` datetime NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
ALTER TABLE `process`
ADD PRIMARY KEY (`id`),
ADD KEY `created_date` (`created_date`),
ADD KEY `exchange` (`exchange`),
ADD KEY `market` (`market`);
ALTER TABLE `process`
MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;
I need to work out the percentage change between Ask1 and Ask2. Ask1 information is in another table called a that has lots of rows of the prices for each market and exchange.
This is how the data in table "a" looks like:
Exchange Market Volume Bid Ask Last Created Date
BLAH APL 3000 1.2 1.3 1.3 2017-07-26 16:31:00
BLAH APL 3000 1.4 1.5 1.45 2017-07-26 16:30:00
I need Ask1 to get information from here getting the highest value in the last 5 minutes. So the sub query I got is:
select MAX(last)
FROM a
where a.exchange = process.exchange
AND a.market = process.market
AND a.created_date > NOW() - INTERVAL 5 MINUTE
LIMIT 1
I need to select it and also do a calculation with it so at the moment I have to have the sub query there twice.
The MAX seems to slow down the sub query a lot.
This is what EXPLAIN says:
id select type table partitions type possible keys key key_len ref rows filtered Extra
1 PRIMARY process NULL ALL exchange NULL NULL NULL 272 99.63 Using where; Using temporary; Using filesort
2 DEPENDENT SUBQUERY a NULL ref market,exchange,created_date market 47 cms.process.market 1603 0.17 Using index condition; Using where
3 DEPENDENT SUBQUERY a NULL ref market,exchange,created_date market 47 cms.process.market 1603 0.17 Using index condition; Using where
The "a" table has about a million rows and "process" table has about 630 rows.
Is it possible to speed it up?
Thanks in advance for your help.