So I am calling the wistia script with a script tag in my head like this:
<script charSet='ISO-8859-1' src='//fast.wistia.com/assets/external/E-v1.js' async defer data-script='wistia' />
However, when I check out the network tab on Chrome, I notice that the E-v1.js script from Wistia is being loaded twice, which is rather significant as it is a 273kb script.
The first load of the script is from https://fast.wistia.com/assets/external/E-v1.js, the location to which I have called it.
However, the second load of the script comes from an iframe, despite me not having put any iframes on the page. This iframe calls the script even on webpages which do not contain any wistia videos. The referrer is: https://fast.wistia.com/embed/iframe_shim?domain=com.
What's going on here? I assume this is some trying-to-be-helpful behaviour from wistia to lazy load their script via an iframe, but it's already loaded...
So I contacted Wistia and got an answer. Their development practices are not exactly intuitive.
Here's what the guy said:
The iframe_shim is a way of tracking the visitor_key for stats tracking, and storing that information on the fast.wistia domain rather than your domain. For a more lightweight method of doing that, you can set window.wistiaIframeShim = false in script tags on your page, and that will stop E-v1.js from loading again. Visitors will then be tracked via a cookie and localstorage directly on your domain instead of the fast.wistia.com domain. As far as I know this shouldn't be problematic, and we'll eventually be changing how that works to make it more efficient, it just hasn't been prioritized yet.
So they seem to load it twice from two different origins just to store a tiny amount of information on their own domain rather than on the client. Seems ridicuous to me, but I can confirm as of right now that all you have to do is change that window variable.
THE FIX: window.wistiaIframeShim = false