Fastest way to start SPI read using External IRQ

I am trying to find the fastest way to go from detecting an external interrupt signal to starting a SPI read

My current process:

1. SPI Read function waits on an Event Flag.

2. When External IRQ signal is detected the Callback sets the Event Flag.

The process enters the External IRQ Callback function with almost no delay but there is significant time before it switches to the SPI read function then there is some additional time before the SPI read actually starts.  I am trying to find a way to reduce the delay from when the signal is received to SPI read start.  Any suggestions?

Thanks!