Hello @Artur,
Thanks for sharing your question with the Klaviyo Community.
Our javascript (which is what powers forms like these) loads "asynchronously" -- meaning it won't slow down the rest of your site. This can sometimes lead to a difference of ~1 second between the page loading and the form appearing; this allows your site to load faster overall, and in our experience, doesn't lead to a lower response rate than a form that loads in-sync with the rest of the page.
That said, we're constantly working on ways to make our code load faster, and I'd be happy to share your feedback with our team. If you'd prefer to use an alternative form, you can look into custom-coding a form like this: Add and Customize a Legacy Embedded Signup Form
This would allow you full control over the form's elements, and would require a developer's assistance to implement.