Version:            (using KDE 4.2.2)
OS:                Linux
Installed from:    Ubuntu Packages

With the "wordwrap" option disabled, the composer window in kmail becomes increasingly slow for long, unwrapped lines.  Although the logical line is unwrapped, the composer window (correctly) wraps the physical lines rather than using a horizontal scrollbar.  When the composer window shows a single logical line wrapped over five or more physical lines, text entry becomes extremely sluggish.  I can out-type the cursor by twenty characters or more, which makes it difficult to edit an email with a reasonable length paragraph.

To test, simply turn off the "wordwrap" option in the composer, paste the above paragraph into the composer window and begin typing (quickly, for best effect) at the end of that line.

Some observations:

This does not appear to affect the entire window.  Short lines remain relatively responsive to keyboard input.

This is unique to kmail, a text box on konqueror does not display the same behavior.

Please let me know if I can provide more information.