Hello,
First of all, thank you for your purchase.
When running importing using the plugin, the slow loading speeds can be experienced only from the browser where you started the importing process, because it waits for the process to finish.
Other visitors of your website will not experience these slow load times.
To check on this, please start importing in the plugin and start a new incognito windows of Chrome (separate from your original browser where you started the importing process). This new incognito browser will be able to load your site without any speed issues.
Let me know if this helped.
Regards, Szabi – CodeRevolution.