Since the "speed" is not an element of the offense you were charged with, the speed information is irrelevant to the citation you were given.
With that being said, there are two ways to look at the seat belt charge. One would be:
The vehicle was not moving when the officer observed you without your seatbelt, so the legal requirement to have it on does not apply. The other way is that the vehicle was moving prior to the stop, and not having your seat belt on when he observed you might suggest that you didn't have it on before (in other words, why would you take it off if you were still seated in the same spot you were sitting in?)... Although without him actually seeing that you didn't have it on prior to the stop might imply that the citation should not have been issued. This all depends on how the statute is worded.... So what statute (number) were you charged with?
Others may have other opinions.

