Comment 1 for bug 571628

This is inevitable due to the way the alternate install CD works; constructing the installed system by installing packages one at a time is much slower than constructing it by copying over a filesystem tree, but also significantly more flexible. We ship both because of this trade-off.

See also:

  https://wiki.ubuntu.com/LucidLynx/ReleaseNotes#Performance%20regressions%20with%20ext4%20under%20certain%20workloads

... and there's a separate bug about that, but even if this particular issue were fixed then the alternate installer would still be inevitably slower, hence the "Won't Fix".