Slow checkout speed

At the end of the checkout, HikaShop gives to your email server two emails, the order creation notification and the order admin notification.
The email server can sometimes take a lot of time to process the emails. Especially if you use the invoice PDF plugin which can attach the PDF invoice to the email.
The process at the end of the checkout shouldn't take more than 10 seconds.
If you use the Business edition, you can go in the menu System->Emails of HikaShop and turn off all the emails. Then, if it works normally and process rapidly the pages, it indeed means that it comes from the email sending process and I encourage you to contact your hosting company so that they can give your the most appropriate settings for the email sending settings of your joomla configuration.