In one moment of web page life - there is some bottleneck. It could be not using compression, a slow DNS response, a big size of jpg or larger picture format (example Jpeg optimization tools, wrong SSL/TLS settings, or missing and similar.
Very handy tool working really well.
Pingdom Website Speed Test
This tool is different than the previous one. It shows many things and recommends + it has different locations to run the test.
It has different tools for checking speed, how content is loaded as also giving tips to improve speed.
Swiss knife for SSL/TLS and gives recommendations on how and what to improve SSL/TLS settings.