The real hits to performance do not come from "I did X writes" or "I did Y reads", but rather from situations such as "I did X writes interleaved with Y reads, which caused Z reflows". Or "I needed to update a tiny part of the screen but triggered a full-page repaint". Or "I'm doing complex animation X, but am not triggering hardware acceleration."
Reflows are very expensive. So are repaints. Chrome dev tools will allow you to detect issues with both of those.
There are a couple of excellent videos on developers.google.com that dig into this a little bit. I highly recommend watching the following video where two Google developers analyze some real world sites and fix the performance issues: http://www.youtube.com/watch?v=z0_jD8nO5Zw There are some great tips in it.
Reflows are very expensive. So are repaints. Chrome dev tools will allow you to detect issues with both of those.