Fast External Interrupt

I need to implement a fast external interrupt with a C167CR micro on a Phytec phyCORE-167 board.
At the moment, I have my external interrupt signal connected to pin P2.15
and program it as follows:

      EXICON = 0x4000;    // Set Ext Trigger Interrupt for rising edge
response
      CC15IC = 0x4c;              // Enables Ext Trigger Interrupt, Global
Level = Lowest, Priority Level = Medium
      IEN = 1 ;                         // Enable Interrupts

When an interrupt occurs, I then want to set another port pin which is done
within the interrupt handler, as shown below:

     void Boxcar_irq (void) interrupt CC15INT = 31
     {
         CC15IC = 0x0;                // Disable further interrupts
         EXTTRIG_GATE = 1;     // Sets pin P2.1 High

         return;
     }

This arrangment works, i.e. the interrupt is trapped and the port pin set
low, but the response time I am getting is 5.4uS.
I have measured this as the time from the rising edge of the external
trigger signal to the rising edge of P2.1.
I do not have any other interrupts running or enabled so I don't set this
interrupt to highest priority.

Can you suggest a way that I can improve the interrupt response time.
I would like to get below 1uS if possible.

My application program is approx 20kB in size and runs from external RAM using.

Any ideas?

Best Regards,
Kieran Dobbyn