does this count as a feature request?
Wouldn't that be super handy so you could see if all your tweaking is working to enhance render times?
The render times are an estimate based on the most recent few frames. The predictions can be way off. The final render time is reported, but a render time vs frame graph is not. To really analyze what's going on with a project and where the hang ups are, that's what would be required. Average render time per frame over time so you could tell what effects are slowing everything down.
I always render with the BG Renderer script. It will tell you how long each frame takes to render in the terminal. It will also give you overall time at the end. It automatically creates a text log of all of this info in your project folder, so you can refer to it at a later time and compare renders. It costs money, but it's well worth it.
If you're still running CS5, a former student of a friend of mine, wrote this script and it's free.
In Andrew's own words it might be a better option than the BG Render Script. Here's why:
Essentially just another way to render in the background (inspired by Lloyd Alvarez’s great BG Render script on aescripts.com), but without ever having to open AE. Render speeds should also be slightly quicker this way as well since no resources are used by the app and only the render engine.