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

If you see this message popping up when bulk optimizing your images (on the menu Media > Bulk ShortPixel), there are a number of possible reasons why it is showing:

• The plugin is attempting to optimize some very large images and the process itself takes an unusual amount of time.
• The internet connection to your website is poor and this means that the whole downloading and uploading process from and to your server takes longer than usual.
• Your website is protected by a firewall. Make sure you whitelist our IPs from your security plugins, your hosting and your CDN. 
• You are using Cloudflare. In this case, please check this article.
• You have some restrictive rules on your ".htaccess" file. A quick way to check whether this is the reason for this issue, is to temporary change the name of the file to ".htaccess_tmp" and then try again to see if the plugin works better. After having debugged this, you can change back the name to the original.