What average block time does the Etheruem network try to achieve?
The average block time has varied from ~30 seconds to ~14 seconds according to https://etherscan.io/chart/blocktime.
Does the yellowpaper define a "canonical" average block time that the network try to achieve?
You can monitor the average block time and other neat stats here.
I understand the initial target was 12 seconds but at the time of writing, the average block time is 15s, which I understand is the desired target.
Before the metropolis release block times increased significantly but this was expected (see Ice Age).
