Comment 19 for bug 655831

I think the simple workaround is to uninstall it if you don't need it:

  sudo apt-get purge apt-xapian-index

Getting it to play more nicely could be difficult; it already uses both nice and ionice from what I can see.

A default install of Lubuntu 12.04 (as of the 2012-03-23 daily build) does not install apt-xapian-index, so this issue should not happen in Lubuntu 12.04, unless people explicitly *choose* to install that package.