End to end request processing time

In a simplistic view, in a typical online system, this is where one needs to check to know any slowness

Browser rendering time - if the page size too big or too complex with java scripts and style sheets
then the rendering time could take time..
# of static content being used on the web page has an impact on the overall load time. And, if they are not cacheable, the round trips even from a CDN could have an impact on the load time
Total download time for a page depends on how big is the response content and how many susbrequests are triggered part of the page and the network time and entire server side processing time
Page load time is the time by when user sees the page.. so above all have impact on it.

coming to network, it is important to know what is the path the request is traversing from client to the server. Is it taking longest path via CDN or how the addresses being resolved over public internet and any proxy being used etc., the network delay and packet loss are important factors to keep an eye on.

coming to server side, the request processing time depends on many factors.. it depends on underlying infrastructure, resources, architecture, code logic implementations etc.,
but there are few check points to look at to break it down..
checking at http server layer tells the variation between end page load time and total server time. This helps identify any network delays.
checking the difference between http server to app server times tells if there is any delay in the middle layers like authentication routing..
on the app server side, it could spend time in many places.. the routing can happen to many servers or even to external systems. using runtime instrumentation tools, it is possible to break down the time spent in pure code, wait times due to synchronous code blocking, gc pause times, time spent in reading from sockets/wirte to while interacting with DB or in the calls to other servers while making remote calls like rjvm to EJBs or service calls etc., by breaking this way, each underlying activity and the delays can be identified

Each of the above ones are not more than just an index to an ocean of tuneable metrics that each underlying technology modules contains. However, where to look at and what to tune is the key.