New feature!
Now the lab gives you a detailed look at what's taking space in your javascript files.
"Flarum Core" and "Flarum TextFormatter" are the base components of Flarum, so you can't do much about them. Then you'll see each extension sorted from the bigger impact.
We can notice that there's rarely one extension having a huge impact (except Password Strength and EmojioneArea at 830 and 650 kB respectively). But it quickly adds up when you have many extensions.
Extensions with a triangle are those that were not compiled for production before being published. If the author re-compiles the extension for production it will take less space. @askvortsov is already on the case for his extensions by the way.
The lab is not going to offer any recommendation regarding assets size. It's up to you to decide what you do with this information.
I plan to also add details about the language and CSS files in a future update.