You may want to use migrate for this but if it's a simple export and import then this module allows users import through CSV files.
Also Feeds is also a good way of importing users now its ready for use in D8, you can quickly set up a parser for CSV and import.
Feeds d8 module
Heres the documentation for ...
Performance is typically due to the Entity API system operations and the complexity of your migration.
First thing would be to ensure you have nothing enabled like XML Sitemap, Search API / Search indexing, Rules, and other operations that happen on node/entity save unless absolutely necessary (like Pathauto for example).
Second, try leveraging Migrate, ...