How can I determine which Private PPAs are slowing down updates?

I've several PPA installed on my system. Every time I perform an apt-get update there's a moment in which the updating process remains stuck in this position (it lasts almost a minute):

Ign https://private-ppa.launchpad.net precise/main TranslationIndex
Ign https://private-ppa.launchpad.net precise/main Translation-it
Ign https://private-ppa.launchpad.net precise/main Translation-en
100% [In lavorazione]

(I don't know in English how the last line is, something like "100% [Working]")

I presume this happens because a download of a PPA is slow. (Do you think this might be the reason?)

If so, I'd like to remove this PPA from my system. The problem is that during this phase I can't distinguish among PPAs because all of them show this URL: https://private-ppa.launchpad.net precise/main. Is there a way to identify the PPA which is being updated in that particular moment?

Use sudo apt-get -o Debug::Acquire::http=true -o Acquire::Queue-Mode=access update. This enables debugging options to make all HTTP requests sequential and show you every HTTP request going out and response coming back in.