@joannaritsoni if you are testing their demo URL directly on GTmetrix the results could vary if you are testing it on a different server as the theme’s developer could be using other tools to improve their performance along with an optimized hosting or server. It is wise to test it first before purchasing but it’s also good to do the optimization work. For instance from this blog post.
Hi @medellin9 did you try also switching the theme? if you’re not using a default one like Twenty Twenty-One, you may grab it from https://wordpress.org/themes/twentytwenty/ and enable it. You could also enable WordPress Debug Mode to see any other technical details.
Hope this helps.
Jacobo.
