Yes, this is know behavior for some shared hosts. This is because your browser is waiting for a response from your server, and it times out.

Also, on further accesses to your page, your webpage might be still in ‘timeout state’. Relax, this will affect only the browser session that you are using. Other visitors and other devices (computers or phones) visiting your website will see it as usual – no timeout for them.

After a while (when the plugin finishes importing posts), the webpage will be accessible also for your browser session.

The timeout is set by your hosting company – and cannot be changed.