Unresponsive for a long time after checking for updates

update-manager 1:0.185, Ubuntu R

1. Wait until dozens of updates are available.
2. Launch Software Updater.
3. Wait until it finishes checking for updates.

What happens:
* Software Updater is unresponsive for a long time before showing that updates are available. For example, on my MacBook with 303 updated packages available, Software Updater is unresponsive for 3 minutes 45 seconds. (That's almost ¾ second per update!)
* The progress bar is full for the whole period.
* After briefly saying "Finished", the progress text reverts to "Downloading from dl.google.com" for that whole period, which I'm pretty sure isn't true.

<seb128> the computation to sort depends from an app under the app icon seems to be quite slow

What should happen: Any combination of progress bar and progress text should never show the same thing for more than about five seconds at a time.

I can think of several complementary ways to reach that goal:
a. Make the processing of updates faster. It shouldn't take 0.74 seconds per package.
b. Maybe process updates incrementally while the repository indexes are being downloaded. (Drawback: this would involve doing work to process updates that turned out to be trumped by yet-to-be-checked repositories.)
c. If the final processing is still likely to take more than about five seconds, pre-allocate a fraction of the progress bar to that subtask, and add incremental progress text, e.g. "Analyzing available updates (1/303)".