Hey. I'm not sure if this is exactly what you want but you can give this a shot.
It will slightly slow the player down during fastrun.
Here are some cvars:
amx_fastrun_slow_amount "3" //Amount of times client will be slowed down during fastrun (int)
amx_fastrun_slow_percentage "40.0" //How much slowdown percentage? (float)
You can increase either percentage or amount to have more dramatic slowdown and vise versa.
Also @Supremache do not change client cvars without their permission....
That has nothing to do with the request.