I have made some path changes over the years, so some images are now in differend folders. To fix broken images in posts, i have done this in htaccess

Redirect 301 /forums/public/style_images/ /public/style_images/
Redirect 301 /forums/images/ /images/

Question (maybe stupid one), does that make page creation any slower?

No, the page generation will be the same speed. In theory the page may load slower for the end user because the browser requests the image from the original URL and then is redirected to the new location (so two requests instead of one), but it will likely be negligible in most cases. Your alternative is to go through all the content and replace the paths instead.

Thanks 🙂 Some SQL query might fix this too.