What is the simulator resolution you have got set? the default is 100ps iirc.
But why are you trying simulation at different clock speeds? simulation is mostly used for functional simulation/bug fixing where you are simulating your code (not a gate level netlist) so the clock speed is unimportant.
Surely you know what your target clock rate is based on data throughput requirements or available clocks on the board? If you meet this clock speed in timing analysis then you should be good to go (assuming you followed good practice, have good timing constraints)
Simulating at different clock speeds is not really going to be very benefitial.