This is a common issue. Sometimes after modifying any .js file in magento cache related to JS are not cleared correctly. So we need to clear it through the terminal... Please run the following commands:
php bin/magento setup:upgrade php bin/magento setup:static-content:deploy php bin/magento indexer:reindex
If you made any change from the admin panel to the configurations setting, like the customer account or other setting, you need to clean the cache or if it is asked, reindex as well.
This can be down directly from System > Tools > Cache management or if you have access to the filesystem and your website, you can try to run this commands:
php bin/magento cache:clean
and if a reindex is required:
php bin/magento index:reindex
As usual Magento checkout page is the most overloaded with JS, that is why it is so slow. There are also other issues that influence load time:
- complex network code
- extra price rules
- useless shipping and payment methods etc.
Here is the guide that will help you cope with all these difficulties: https://onilab.com/blog/magento-checkout-slow-fix-it-in-8-simple-steps/