Unexpected errors happening after installing Dispatch

Hi there,

1. That space is coming from the Auto Ads being placed above the header.
2. You site is disallowing PHP Editing, and the Dispatch site requires PHP in the GP Hook Element.
Add the PHP Snippet provided here to allow that:

https://docs.generatepress.com/article/generate_hooks_execute_php/

Adding PHP:
https://docs.generatepress.com/article/adding-php/

3. The Dispatch site home page is a heavier site then a standard blogs home page as it loads more content then your standard blog. For example the site loads by default 23 posts with images. So an increased load time is expected.

can I use the default jetpack related post in the theme?

You can, but JetPack is not lightweight.