One problem we had was that while iframes already do load asynchronously, the main page will not fire OnLoad until the iframe has finished loading too. iframes should load asynchronously without any effort on your part.

Is there a way to load an iframe asynchronously?

They certainly can. In Chrome 77, Chrome added support for automatically lazy-loading offscreen images and iframes when a user has opted into Lite Mode (Data Saver mode) in Chrome for Android. Every byte matters and so lazy-loading iframes has the potential to make a meaningful difference for these users. ...

Do iframes slow down page load?

Every iframe on a page will increase the memory used as well as other computing resources like your bandwidth. ... To avoid having your iframes slow down your pages, a good technique is to lazy load them (i.e., loading them only when they are required like when the user scrolls near them).