characters display slowly when serial is enabled and debug level >=9

[Iru Cai]

Hello,

I just found that when I enable serial output and set debug level >=9,
characters will display very slowly. It becomes worse when the machine has
serial output. I can see the characters show on screen one by one.

After I see the serial output. I think handler_16 is to blame, because
there are many output for it.

Iru