Odd delay in execution of u

When undoing the most recent change in insert mode by typing
<C-O>u , there is a delay of about three seconds before the undo
takes effect.

For example, start vim like this,

$ vim -N -u NONE

and insert two sets of changes by typing the following:

first change<Esc>A second change

where <Esc> is a literal Escape. Now undo the second change by
typing

<Esc>u

The second change is deleted immediately. Now add the second change
again, but this time undo it while remaining in insert mode by
typing

<C-O>u

Notice that the cursor moves to the start of the second change
immediately, but that there is a pause of about three seconds before
the text of the second change is deleted.

Why is that, and can it be fixed?

I'm using Vim 7.4.838 in an xterm on Fedora 14.

Regards,
Gary

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