SPX, which stands for Simple Profiling eXtension, is just another profiling extension for PHP. It differentiates itself from other similar extensions as being:

totally free and confined to your infrastructure (i.e. no data leaks to a SaaS).
very simple to use: just set an environment variable (command line) or switch on a radio button (web request) to profile your script. Thus, you are free of:
manually instrumenting your code (Ctrl-C a long running command line script is even supported).
using a dedicated browser extension or command line launcher.
multi metrics capable: 22 are currently supported (various time & memory metrics, included files, objects in use, I/O...).
able to collect data without losing context. For example Xhprof (and potentially its forks) aggregates data per caller / callee pairs, which implies the loss of the full call stack and forbids timeline or Flamegraph based analysis.
shipped with its web UI which allows to:
enable / configure profiling for the current browser session
list profiled script reports
select a report for in-depth analysis, featuring these interactive visualizations:
timeline (scale to millions of function calls)
flat profile
Flamegraph

The goal of any great website is to provide quality content for its readers and clients, though no website can be truly successful if it cannot provide the content in a timely manner. How much time is acceptable isn’t always up to you though. It’s in the eyes of the beholder, hence the readers.

If you have a flash based website that is media heavy, and your users knows is, they will probably be ok waiting a minute or two while your page loads. Though, if on the other hand you have a mostly text based website and it takes two minutes to load, there is 99.99% chance the person will not wait. These days, people want information and websites to be practically instantaneous.