OK I could apply a 0.025 scalar to the hp speed which would have the effect of making is so that the speed scale when there is 20 zombies alive is 50% less than current and when there is only 5 zombies the speed scale is 87.5% less than current. In that your min speed would be 50% faster or 87.5% faster than currently under this math.

I will get this programed and I think it will resolve the issue you are having.