Easy Country blocker app does not slow down any store because this app injects the script as async and scripts are hosted on CDN. So you never face any issue like this.
The app is well-designed and optimized for performance, it should not slow down the store anyway. users can feel free to use the app and they will not face any kind of problems.
It’s also worth it that blocking traffic from certain countries could never reduce the load on your server and improve the overall performance.
In general, it’s a good idea to use the app or its feature then you can choose the better option for your store.
Install the app from here.
Shopify: https://apps.shopify.com/country-blocker-1
Bigcommerce: https://www.bigcommerce.com/apps/easy-country-blocker/