float chill_effect = 0.2;
Stat stat = {.base = chill_effect};
// ...

// This will totally stop the enemy if final_effect is >= 1
float final_effect = calculate_stat(stat);
enemy_speed *= 1 - min(1, final_effect);

// This makes increasing chill weaker and weaker. Will never completely stop the enemy
float final_effect = 1 / (1 + calculate_stat(stat));
enemy_speed *= final_effect;