Hi,
Thank you for your answer.
Yes I did allow for svg in the image field. Other svg files uploads fine, only slightly heavier one seems to pose problem.
The svg has been exported with Adobe Illustrator which does indeed produce bloated svg. I used an online compressor on the problematic file (https://vecta.io/nano) and it did upload after that. I'll look into the plugin you linked, thanks !
Also, the general slowness of the admin page seems to be gone when I put the website online, so it may have more to do with my local server settings or something.