Is it possible to speed up a sum() in MySQL?

Turn on QUERY CACHE in mysql. Caching is OFF by default. You need to set mysql ini file.

-- hint mysql server about caching
SELECT SQL_CACHE sum(foo) FROM bar;

MySQL optimizer may be able to return a cache if no changes were made to the table.

Read more here: http://www.mysqlperformanceblog.com/2006/07/27/mysql-query-cache/

Two things here:

1) You should not do sum for 7.3m records on regular basis - introduce staging tables serving business needs (by day, month, year, department, etc.) and fill them on scheduled basis, possibly reuse those tables instead of original 'raw' table (like select summarized value for each day when you need few days interval, etc.)

2) check your transaction settings

http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html#isolevel_repeatable-read

No, you can't speed up the function itself. The problem here is really that you're selecting 7.3 million records. MySQL has to scan the entire table, and 7.3 million is a pretty big number. I'm impressed that it finishes that fast, actually.

A strategy you could employ would be to break your data into smaller subsets (perhaps by date? Month?) and maintain a total sum for old data that's not going to change. You could periodically update the sum, and the overall value could be calculated by adding the sum, and any new data that's been added since then, which will be a much smaller number of rows.