Serial console causes lots of interrupts on RPi Zero

Hi,

I've got a Raspberry Pi Zero which is configured to use the serial console 
at boot (no "console=fb" in /boot/cmdline.txt).

The system seems slower than other first generation and Pi Zero devices. 
While running top, there's a constant 11% or higher interrupt load, and 
sysstat vmstat shows:

  31648 Interrupts
    100 bcm2835 p timer
        bcm2835 p Mailb
  23004 bcm2835 pi uart
        bcm2835 pi dma0
        bcm2835 p sdhos
   8544 bcm2835 pic usb
        bcm2835  Doorbe

That's a lot of interrupts!

With the serial console turned off at boot, things look a little nicer:

   8759 Interrupts
    100 bcm2835 p timer
        bcm2835 p Mailb
        bcm2835 pi dma0
        bcm2835 p sdhos
   8659 bcm2835 pic usb
        bcm2835  Doorbe

There's only around 2% to 3% interrupt load, and everything runs about 15% 
faster.

Does anyone know why interrupts go crazy when the serial console is used 
at boot?

John