Open up your config.cfg file and modify these two lines:
cl_forwardspeed "400"
cl_backspeed "400"
Use a value below 400... You may have to try a few times until you get the exact speed you want
You might also want to add in this line:
cl_sidespeed "x"
where "x" = the same value you choose for the above mentioned two lines.
And be sure to make a backup copy of config.cfg in case you screw anything up