Images rescale taking forever

Hey everybody !
I will do my best with my english, please forgive me ??

Here’s my issue:
I was doing a little project a few months ago, that I just restarted today. But I can’t explain why, when I upload an image to the gallery, it’s taking like ~5mn to upload it & rescale it. The issue seems to come from the rescaling.
Meanwhile, everything is laggy, even the blog online can’t load. As if the CPU or something of the server was dying. I’m not quite comfortable with coding & stuff, I just try to do my best to learn online, so I guess I’ve done something wrong.

Here’s what I tried:

• Switch back to the basic Twenty Seventeen theme;
• Desactivate all plugins & stuff;
• Rollback to an old wordpress version;
• Change every settings of image rescaling;
• Try some image rescaling, etc, plugins;
• Change the CHMOD of the folders;
• Upgrade the Max Memory Limit in php.ini;
• Reupload a fresh WordPress install (except from wp_content);
• All of those at the same time :D;

Nothing changed.
But I’m missing something stupid I guess. Image format, size, nothing matters, it still takes forever.
I’m running WP 5.3, Minimal Portfolio 1.0.8 as theme, & Filterable Portfolio 1.5.0 as my only plugin. But even without those it doesn’t change anything.

There’s one thing that bothers me:
The image clearly uploads directly. I can see it in Filezilla. But even if the rescaling goes fast, I can’t see the thumbnail in the gallery, in the post, nowhere. & so, I can’t add it & publish anything. The thumbnail is just blank.

Thanks a lot for any help or ideas you guys can have. Sorry about my english & my coding skills..