Is the sizing chart or button showing up too late? (seconds or even 10s of seconds after the page load).
The most common reason for the slowness is because the script installed by Kiwi (and most other apps) are loaded "asynchronously". Essentially it waits for the whole page to load first before loading those script. And if the product page has a lot of content to load (i.e. images), the kiwi script would only be triggered after all these content are finished loading, which could take quite a while.
The solution for this issue is to load the Kiwi script much earlier on the page.
Inside your layout/theme.liquid and put the following script right after the "</body>" tag.
<script src="https://app.kiwisizing.com/web/js/dist/kiwiSizing/plugin/SizingPlugin.prod.js"></script>
If you are unsure where and how to do this, please reach out to us and we can work together to add the script.