After you install the mod after each successful minecraft start you will see a file called "loading-log.log" in your minecraft root folder. If you open that up you will see something like this: Pastebin. In that file you can see how long each mod took for the respective fml cycles (Constructing,PreInitialization,Initialization,PostInitialization) in ms.
Since 1.1 there's additional data logged that shows how long the class transformers of the different core mods took. Notice that the class based log might not be the most useful / reliable.
All data is also logged to the debug log.
