It's not uncommon to experience slower site speed after integrating Klaviyo with your WooCommerce platform. Many users have reported similar issues, particularly related to the size and loading speed of the JavaScript files associated with Klaviyo's features, such as signup forms. These scripts can add significant load time, especially if they are not optimized for performance.

The Klaviyo JavaScript files, especially those for embedded and popup signup forms, can contribute to longer load times. For instance, users have noted that certain Klaviyo scripts can take substantial time to load and may be blocking the main thread, which impacts overall site performance​

I have also noticed the teaser for the signup form is not automatic. It takes a few seconds for it to populate. 

Klaviyo review codes typically involve JavaScript that interacts with your WooCommerce site to collect and display review data. If the script isn't optimized for performance, it could potentially slow down page loading times.
 

Regard,