I just discovered the informative blog-post Optimize the Performance of Widgets, Buttons & More on w3-edge.com.
It’s a well known problem that 3rd-party services included in a web page can become a performance bottleneck.
The article explains how to defer the loading of 3rd-party widgets (like google ads, social network plugins etc.) until after the HTML is rendered. It gives a lot of code examples for the most popular addons (also for some that I didn’t hear of). These should enable you to add the JS functionality after the page has been loaded for these elements.