Huh? What makes you think its not going at the right speed? I presume your talking about a case fan. In which case if its an Antec they do have a speed controler attached. Some other brands do have speed control too.
If its a CPU fan or the case fan in plugged into the motherboard and not the PSU then its possible that your Motherboard is automatically slowing down the fan as it thinks your PC is cool enough that you don't need it on full speed.