Hi there,
Can your host be more specific about what in the theme is causing it? There really isn’t anything in the theme that would cause your site to slow down. If there was, it would affect all sites using GeneratePress (including this one).
When I ran your site through Pingdom, it seems like you’re loading a lot of third-party APIs (Google fonts, wp.com).
Besides that, the biggest issue I see is the “Wait” time, which is typically your server.