If an image with Swedish characters (åäö) in the filename is used, the article is saved superslow and the page end up with an 500 error.

And even stranger is that it only occurs for Publishers and not for Supersusers like me....

This is because the issue is caused by the Joomla Text Filter processing the content on saving. The Text Filter settings for Super USer are probably set to No Filtering, but for the Publisher user group they are set to Default Blacklist, so the content is filtered when saving. This process appears to be stalling on content that contains special characters in attribute values, like the url in the img tag src attribute.

This has been reported and appears to be on the way to a fix, perhaps in the next Joomla update - https://github.com/joomla/joomla-cms/issues/39515