external services: analytics, ads, trackers, social. Loading an external service can add over 900ms to your load time. Synchronous Javascript calls will block your page load.
until all CSS is downloaded. Browsers will download all CSS before rendering, even if it is unused. Does your mobile phone really need to download the print stylesheet?
cloud are abundant but inconsistent. Failing resources create a domino effect of further failures. Scaling has more to do with people more than any other resource.
add up quickly, even within the same datacenter. Browsers will limit simultaneous downloads from the same host. Service Oriented Architectures are pushing more data through the network.