Jack,
I see no one has replied so I though I would say something. I am using the DSP563xx processor series and if you read the datasheet it tells you a bit about the fast and slow interrupts. There is a big difference and I start by explaining the interrupt process.
Each interrupt has a two instruction slot in an interrupt vector table. When an interrupt occurs, the program counter is frozen and these two instructions are fetched and executed. Normally a jump instruction is inserted in the table which jumps to the ISR so the interrupt routine can be executed. This is a long interrupt.
A fast interrupt is used if neither of the two instructions are a jump insturuction. Here, the two instuctions are executed, and the program continues. The benefit of a fast interrupt is that the state of the machine is not saved and all the overheads associated with a jump are not there making it very fast.
So basically, if you can execute your interrupt in only two instructions, then a fast interrupt can be used. Here, you may only set a flag, or use some special instructions that have been designed so you can transfer data between peripherals.
For more information, check out the DSP56300 Family Manual and look at section 2.3.2.
I hope that helps.
Cheers,
Carl