There are size-limits on ads. Actually there have always (read: for the last 10+ years) been limits. Also, ad servers are usually rigorously tested with regards to performance, speed etc.
Not all sites enforce these limits, but major publishers are very aware of users' interests.
The problem is that if you visit an average website, there will probably be 20+ servers involved in serving the page, the javascripts, analytics, ads, etc.
One of those systems occasionally underperforms, which would not be a problem if everything but the main content was simply loaded asynchronously, possibly in the order of importance a website owner attaches to each content type.
Not all sites enforce these limits, but major publishers are very aware of users' interests.
The problem is that if you visit an average website, there will probably be 20+ servers involved in serving the page, the javascripts, analytics, ads, etc.
One of those systems occasionally underperforms, which would not be a problem if everything but the main content was simply loaded asynchronously, possibly in the order of importance a website owner attaches to each content type.