Poor listbox performance in Tk 8.5a6

The following simple script demonstrates a siginificant listbox performance regression in Tk 8.5a6:

pack [listbox .lb]
puts [time {
for {set n 0} {$n < 100000} {incr n} {
.lb insert end "This is item #$n"
}
}]

On my rather slow Linux box (running SuSE Linux 10.0), this script produces the output

1793963 microseconds per iteration

Compare this with the time needed by Tk 8.4.13:

1126718 microseconds per iteration

This is an unacceptable performance regression.