--http://onlyjob.blogspot.ru/2011/03/perl5-python-ruby-php-c-c-lua-tcl.html
io.stdout:setvbuf "no";             --  io.flush();

str = 'abcdefgh'..'efghefgh';
imax = 1024/string.len(str)*1024*1;         -- 1mb

starttime = os.time();
print "exec.tm.sec\tstr.length";

gstr, i = '', 0;

while i < imax+1000 do
    i = i + 1;
    gstr = gstr..str;
    gstr = string.gsub(gstr, "efgh","____");
    lngth = string.len(str) * i;
    if (math.mod(lngth,1024*256) == 0) then
        print(os.time()-starttime.."sec\t\t"..(lngth/1024).."kb");
    end
end

C:\Lua_tests>lua-5.1.4\lua.exe test514.lua
exec.tm.sec     str.length
29sec           256kb
117sec          512kb
264sec          768kb
469sec          1024kb

C:\Lua_tests>lua-5.2.3\lua52.exe test523.lua
exec.tm.sec     str.length
30sec           256kb
118sec          512kb
272sec          768kb
485sec          1024kb

C:\Lua_tests>lua-5.3\lua53.exe test53.lua
exec.tm.sec     str.length
34sec           256.0kb
139sec          512.0kb
319sec          768.0kb
568sec          1024.0kb