I tried to revert but v39 is now the latest release. It's worth adding that I'm running the x64 version of chrome. Here are my complete chrome beta / stable results:
Chrome v39 (Version 39.0.2171.27 m (64-bit))
19.1 - 25.2 ms; however, there are spikes to a max of 39.7 ms every 5 seconds (lasting 2 seconds)!!
Chrome Beta (Version 39.0.2171.27 beta-m (64-bit))
19.3 - 25.7 ms; the same spikes to a max time of 43.1 ms every 5-6 seconds (lasting 1-2 seconds).
Edit to add - judging by the version numbers, it seems that I should expect very little difference between these tests (QED).