Adrian,
I do not think you are seeing any jitter due to the speed of the IO's, it is more likely that your jitter is coming from the correlation engine inside the CC2520 is trying to make a decision on when the SFD has arrived. This happens at a certain number of time over sampled compared to the data rate and it is normal for the correlation engine to have a jitter of +/-1, which means it will select SDF=-1 or SDF=0 or SDF=+1. This will be presented as jitter on the SFD pin, but with integer step sizes in time.
Regards,
/TA