seriously need help with pulse lightspeed 2 125

Hi everyone :) so I have a pulse lightspeed 2 125, 4 stroke, on my hands, bike runs fine starts fine revs fine idles fine as far as I'm aware there is nothing wrong with it, however it will only do about 40mph top speed and I don't have a clue why it's doing this! New lextek iridium exhaust new inlet manifold gasket relatively new belt and 13g rollers passed MOT. Aby suggestions as to why it's going so slow? Also what are best rollers for it? Thanks!