Sorry to hear you’re unhappy with your PageSpeed Insights score, hopefully I can explain what may be occurring. Without knowing what your site I suspect you may have multiple modules connected. With each Google service connected there are additional requests made to each service. These are the same requests whether you’ve added any Google service manually, as opposed to connecting the modules within Site Kit, the plugin facilitates the addition of each product snippet.
You can also check your sites source code with with just the Search Console module active within Site Kit. You’ll notice Site Kit doesn’t adding any additional performance impacting snippets to your site. The only additional code you’ll find is a meta tag.
If you’d like perform a comparison with manually placed snippets compared to Site Kit placed snippets I’d be happy to check them with you? That way we can ensure there are no additional performance impacting features on your site. If you want to share any details here or privately via this form I’d be happy to check.