[Zenith77]

Instead of using set_task(), why not do something like this:

new bool:gSetSpeed[33];
public FM_PreThink(id)
{
    if(get_pcvar_num(toggle_pcvar) && (pev(id,pev_button) & IN_JUMP) && !(pev(id,pev_oldbuttons) & IN_JUMP) && (pev(id,pev_flags) & FL_ONGROUND))
        gSetSpeed[id] = true;
}

public FM_PostThink(id)
{
    if (gSetSpeed[id])
    {   
        gSetSpeed[id] = false;
        set_speed(id,get_pcvar_float(speed_pcvar))
    }
}

I don't know, maybe it's just personal preference.