Hi there @Ben,
Welcome to the Community!
Have you tried to use the save feature and are you seeing that the blocks do not appear?
You would be able to save your blocks by hovering or clicking on a block and selecting the star icon. This will save the selected block.

I have provided an article for reference on creating and managing saved blocks.
There can be a case where your browser or cookies(cache) could be holding onto a bad or delayed session, causing a hiccup. I recommend clearing out your browser cookies, logging back into Klaviyo, and testing the template again.
But I would also suggest that updating your browser and computer, clearing the cache, restarting your browser and computer and doing an overload of these things may cause speed issues on the platform of your computer. Since Klaviyo is web based, any performance issues on our end would come from internet speeds or servers as opposed to the technology you are using.
Best,
Alex