Hi @orangefloor,
You can try the solution here for increasing your PHP memory limit. But please do mind that these are server configurations, we do not have control over these. If the server configuration limits your site to 256mb then you need to contact the server admin (your hosting) to increase these limits for you (if possible). We are not blaming the host, we are just saying we don’t have control over what you’re asking.
Base on your screenshot, your current server configuration is decent. You said above “We tested with another hosting” how did that go?
I understand your frustration here, as I experience a few sites that are too slow on the backend, but those are not because of the Theme, as I also saw sites that are lightning-fast using the same theme. Its how the site and server are configured makes the difference.
Here is a 3rd possibility and its the common one, you might have a plugin (or plugins) that are not configured properly and it’s eating your server resources, or a poorly written custom code on your child theme that is also wasting server resources, there are a lot of factor when it comes to site performance. Another thing, I see on your screenshot that you have a custom rules on your .htaccess file, that could be in play as well.
Some tips on improving your site performance, It might help.
Performance Tips: Reducing Plugin Load
Performance Tips: Minifying HTML, CSS & JS
Thanks,