How do I speed this up?

Slows down gradually to just crawling:


    while {[gets $fi line] >= 0} {
        incr linenum
        set line [string trim $line]
        if {$linenum % 1000 == 0} {
            puts -nonewline Read:$n:$linenum\r
            flush stdout
        }
        if {[string length $line] > 0} {
            set line1 "$line1 $line"
            if {[regexp {;$} $line] || [regexp {endmodule\s*$} $line]} {
                puts $fo $line1
                set line1 ""
            }
        }
    }