I also created an account on your site and posted a new ad including 1 image. That went pretty good but editing did gave me some 'lag' the first time. I updated the ad again which in itself was ok and then edited the ad again which gave me a 'huge' lag what you seem to experience as well.
Looking in my browser I also noticed the following error:
DOM7009: Kan afbeelding bij : data:image/webp;base64,UklGRhoAAABXRUJQVlA4TA0AAAAvAAAAEAcQERGIiP4HAA== niet decoderen met MIME: image/webp.
It's dutch and says; DOM7009: can not decode image data:image/webp;base64,UklGRhoAAABXRUJQVlA4TA0AAAAvAAAAEAcQERGIiP4HAA==with mime type image/webp
This could actually be a cause for the delay experience and is server setup related, not Osclass setup related.
I also just noticed your site changing theme