How can I view what causes a page to load slow?
Yes you can figure out which function(s) spends more time in the execution by using a profiling module, then you can optimize that particular function(s)

Below are the available profiling modules in Drupal 7(AFAIK),


XHProf
Profiling


Personally I used XHProf and it helped us lot to improve the performance.


  XHProf is a hierarchical profiler for PHP. It reports function-level
  call counts and inclusive and exclusive metrics such as wall (elapsed)
  time, CPU time and memory usage. A function's profile can be broken
  down by callers or callees. The raw data collection component is
  implemented in C as a PHP Zend extension called xhprof. XHProf has a
  simple HTML based user interface (written in PHP). The browser based
  UI for viewing profiler results makes it easy to view results or to
  share results with peers. A callgraph image view is also supported.


In addition to this you can also look into Performance Logging and Monitoring module which was removed from Devel.