15-20 is totally unacceptable. Ads are typically included with a JavaScript file (supplied by the ad vendor) which will load the resources for the ad.
If the script takes long to load it will block all other JavaScript on the website because browsers will wait until all scripts are loaded. In this case you can delay loading of the script file either by adding defer
or adding the script dynamically with JavaScript. See: How to Append <script></script> in javascript?
If the ad resources take long to load it shouldn't impact other aspects of the site. From a UX perspective make sure the ads don't pop in and move other elements when they are finally loaded. You can do this by making sure their space is already allocated.
You can use the Network/Net tab in Chrome/Firebug to see which resources are loading when and see when the browser is read to run JavaScript etc.