How can I use FireBug to tell what's causing slow page loads?

Question

I'm working on a complex page with multiple scripts, css files, lots of dynamically generated html, scripts loading scripts, all kinds of crazy stuff. Naturally, the page can load quite slow sometimes.

I'm finding it difficult to tell, in realtime, what exactly is slowing it down. Can FireBug's "net" tab help me with this? I've looked at the "net" tab, of course, but it seems that it only shows files after they're loaded. Is there another tool that can help me? What I want is to be able to see that the page is still loading, and see exactly what's still loading.

FireFox's status bar tells me a little bit... it says "waitin for www.mydomain.net", but it doesn't tell me exactly which file it's waiting for. I'm assuming there is a single file that's the bottleneck here, that's blocking all subsequent files from loading, but I'll admit, I don't know that much about the nitty gritty of how browsers fetch files.

Answer 1

There is a plugin called YSlow for FF that should help.

Answer 2

On the Console tab - click profile... it will help you pinpoint which functions are running slow.

I also highly recomend the YSlow addon for Firebug. It will help diagnose lots of issues from caching and gzipping to the number of HTTP requests that might be slowing things down.

Answer 3

Something that complicated is bound to have room for improvement. I'm not a Firebug expert, but can you pause the execution and look to see what it's doing and why? If you do that a few times, you should be able to spot time expenditures that aren't really necessary.

Answer 4

There's also Google Page Speed, which does similar things to YSlow.

Answer 5

Your best bet in that case would be something like Fiddler (stand-alone program), Tamper Data, or Live Headers (both Firefox plugins).

Each lets you see the HTTP requests as they come in. These give you real time views of what is being requested and should give you more insight into what is hanging your site up. Good luck!

Answer 6

If you wait until the page does finish loading, FireBug will show you in the Net tab the file that took so long.

I agree that it would be great for Firebug to show all of the currently loading files.