Login to Pingdom and check the tests reports.
Identify reports that are having oddly high load time values. Check test details.
Test details summary. You might see some warning signs already in the test summary, but that is not the case here. The only improper value is load time. Scroll down to Timeline to investigate further.
In Timeline section you can see what resources were requested. Per resource how long did it take to load it. Did resource load succeed and more.
Please note that you can sort Timeline in many ways, but Sort by load order and Sort by load time will be useful to our needs.
Using Sort by load order first, identify major load delays in the graph like shown below. For the resource that caused it check its domain to find out where is it coming from. You can also check details (chevron on the right) to look for more information.
Continue looking for all major delays and identify all resources responsible for those.
Switch to Sort by load time view and review resources that were loading the slowest. Unless those are site images (that is expected), your load time might be suffering due to external requests not finishing in timely manner.
Taking into account the above, the most usual cause of performance issues are Ad providers networks that fail at given time to provide prompt response to requests from your site. On the other hand there are also times when even Facebook requests fail, so the most important is to define who is behind the resource domain for which requests do not return in timely fashion.