Weird speed regression with xft

# This is a variant of http://paste.tclers.tk/3224 in the sense that
# it demonstrates the problem I'm faced with with just tcl/tk core
# (ruling out that the problem is located in the in 3224 used tktable
# extension).
#
# If you run this script with any halfway recent 8.5.x or a 8.6.x
# tcl/tk build under linux without xft support (build with
# --disable-xft) and click on the both buttons the display changes
# almost immediate.
#
# Not so, if you run this script (still on linux) with a recent
# default build with xft support (which is, what you typically have at
# hands). If you click first "Short text" to see something and then
# "Long text" you see how slowly the display changes.
#
# With a standard windows build there isn't such a problem.

for {set row 0} {$row < 30} {incr row} {
    for {set col 0} {$col < 20} {incr col} {
        label .${row}_$col -width 5 -textvariable text
        grid .${row}_$col -row $row -column $col
    }
}

button .ltext -text "Long text" -command {set text [string repeat "abcdef" 100]}
button .stext -text "Short text" -command {set text "abcdef"}
grid .ltext -row 30 -column 0 -columnspan 10 -sticky we
grid .stext -row 30 -column 10 -columnspan 10 -sticky we