Why did upgrading Drupal 6 cause a huge increase in database traffic?
Apart from the NewRelic suggestion, which I put in a comment, consider reproducing this issue in a second environment, I assume you do have a staging/testing environment of the site, or at least a local installation. If not, you should get one ;)

Then you can install the devel.module, enable query logging and/or xhprof profiling. Then you can actually switch between the old version and the new one and compare the difference within Drupal. Devel's query logging will tell you exactly which queries are executed and how long they took.