% proc K {a b} {set a} % for {set i 0} {$i<100000} {incr i} { lappend a $i; lappend b $i } % time {set a [lreplace $a 40 80]} 24879 microseconds per iteration % time {set b [lreplace [K $b [set b {}]] 40 80]} 537 microseconds per iteration