Shouldn't this be in the "Other" or DSP forums?
Read the Data Sheet for the DSP, specifically the "GPIO Timing" section. The timing is relative to FOSC and XTAL which are slower clocks than your CPU. Port F timing is different to all the other ports too.
Check your CPU execution speed. Spin in a loop for 100 cycles, THEN write to the GPIO pin. Make sure the CPU is running at the speed you expect.
Write values directly to the GPIO register rather than BSET/BCLR. They probably have to read-modify-write to the port which is slower.
The GPIO port is on the other side of a bus switch. Check how it is set up.
The GPIO port is probably running from a slower internal clock. See if the Ref Manual tells you which one.
I've used an 800MHz ARM core that could only write to its GPIO ports at about 1MHz too, as the port was on the other side of one or two bus gateways and running on a very slow clock. That meant that a simple read of a GPIO port would stall the CPU for 300 instructions, ditto back-to-back writes to a GPIO port. The manual for that chip strongly recommended using timers and DMA-based peripherals for I/O rather than reading and writing the GPIO ports from the CPU.
