How to update juste one page on my website ?

[Acorn]

I have 25 pages... Each one with lots of photos and texts (it's for a holiday house rental, so we've got 4 pages for the houses, and each one exists in 4 languages...).

Have you Optimised All Images? This might help with the internal RAM usage and generation times.

The Fast Publish of 15 seconds is right. Even if an HTM file is showing it is not necessarily being uploaded.

Give all of your large images meaningful Image Filenames. This will stop them being republished as a different Xara numeric filename each time. I don't think this happening here but it is good for SEO, which you probably need.

I do remedial paid work for Xara Websites and have completed an international one with multiple languages. Generation times were never dramatic, usually 35 seconds but I worked hard on images and MP4 to tweak its ranking with Lighthouse.
You could open your browser Dev Tools > Lighthouse and see what is says, particularly around image file sizes.

Acorn