If the amount of load is not important and you only want to know which elements in your webpage are consuming more time (from a single client perspective) you could take a look at YSlow. It also gives you recommendations on how to improve your load time. In case you need to check this within stress conditions, Silk Performer's TrueLog includes very similar ...
I am only using Selenium WebDriver with Java so I cannot answer your question about JS. But with Java, you can generate your own HTML reports using one of the JUNIT classes, TestWatcher. TestWatcher allows you to override the failed() and succeeded() JUNIT methods that are called automatically when a test passes or fails. You can also override the ...
If you are familiar with JMeter, the you should use the same for load testing the Ajax enriched web applications. You need to use "JSR223 Sampler" for this purpose. You can also go for the licensed tools like VSTS, Neoload, Silk Performer and Load Runner (very expensive).
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