"Lenghty operation in process", what does it mean?

There are a number of possible reasons for this kind of warning:

• the plugin may attempt to optimize some very large images and the process itself takes an unusual amount of time
• the internet connection to your website can be poor and this means the whole downloading and uploading process takes longer than usual
• your website can be protected by a firewall and in this case you might need to whitelist our IPs
• if you use Cloudflare you may want to check this article
• you could have some restrictive rules in your .htaccess - here's a way to test if that's the case.