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I am on Firefox 3.5.1 on Windows XP. I notice that Firefox become very sluggish the moment I have some active downloads. As soon as the downloads are over, Firefox is back to being snappy.

Why does this happen? How do I fix this?

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What is the speed of your internet connection? – Joe Schmoe Jul 20 '09 at 2:09
up vote 4 down vote accepted

Unless this also happens with downloads from slow sites, I'd say there is a good chance your download(s) are saturating your bandwidth, causing page loads to struggle to get through the same channel.

Note: if this is the case, basically the only solution is to use some plugin or separate app for downloads that is capable of limiting the bandwidth used by downloads.

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The other two answers by jerryjvl and Axxmasterr seem to be right on the problem.
I am just adding a firefox-3.5 compatible download manager that can be tweaked to control.

The addon is called DownThemAll.
Lookup the features at the author's howto-features page

  • It pipelines multiple downloads
  • Allows multi-part download -- which you can turn off
  • Allows a control on the number of active downloads from the pipeline

Start with Multipart download setup to 1 segment per download and,
number of concurrent downloads setup to 1 too.
Check bandwidth availability from firefox.
From there either increase to multipart and concurrent downloading, or,
Choose to pause the download while you are changing pages,
depending on your response experience.

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Try using a download manager separate from the built in version. That is how I stopped FF from getting wrapped around the axle.

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