この関数を最適化するには、週末は常に7日ごとという一定の周期で発生するという事実を利用すればよい。そこで、日付範囲内のすべての日を繰り返し計算する代わりに、日数を7で割って小数点以下を切り捨て、その範囲内で発生する週末の数を計算します。そして、その結果を2倍し（週末は1回につき2日あるため）、範囲の最初または最後に発生する余分な週末を追加します。これが最適化された関数です：

 ```
import datetime

def count_weekend_days(start_date_str, end_date_str):
    start_date = datetime.datetime.strptime(start_date_str, '%Y-%m-%d').date()
    end_date = datetime.datetime.strptime(end_date_str, '%Y-%m-%d').date()

    num_days = (end_date - start_date).days + 1
    num_weekends = (num_days // 7) * 2
    remaining_days = num_days % 7

    for i in range(remaining_days):
        date = start_date + datetime.timedelta(days=i)
        if date.weekday() == 5 or date.weekday() == 6:
            num_weekends += 1

    return num_weekends
```
To optimize the function, we can take advantage of the fact that weekends always occur on a fixed cycle: every 7 days. So instead of iterating over every day in the date range, we can simply calculate the number of weekends that occur within that range by dividing the number of days by 7, and rounding down to the nearest integer. We can then multiply that result by 2 (since there are 2 weekend days per weekend), and add any extra weekend days that occur at the beginning or end of the range. Here's the optimized function:

 
import datetime

def count_weekend_days(start_date_str, end_date_str):
    start_date = datetime.datetime.strptime(start_date_str, '%Y-%m-%d').date()
    end_date = datetime.datetime.strptime(end_date_str, '%Y-%m-%d').date()

    num_days = (end_date - start_date).days + 1
    num_weekends = (num_days // 7) * 2
    remaining_days = num_days % 7

    for i in range(remaining_days):
        date = start_date + datetime.timedelta(days=i)
        if date.weekday() == 5 or date.weekday() == 6:
            num_weekends += 1

    return num_weekends