Use real browsers - both with artificial testing environments like webpagetest.org and Real User Monitoring to see what your users actually experience. I won't recommend a specific RUM tool as there are a bunch of options but they all use JavaScript to get basic information and the new Navigation Timing API to get more detailed values:
One word of caution: ignore tools like Google Analytics which are designed by the statistically ignorant. If a tool uses the mean or an average rather than a median or n-th percentile, you cannot trust it to make decisions because real-world timing data has staggering outliers – 99.999% of your users may load a resource in less than a second but the two clients which reported 2 hour load times will skew the average so much that you'll think you're seeing a problem which doesn't actually exist.
I wrote about an example of this in 2012 and despite the assurances of people who work on Google Analytics and, I'm told, internal lobbying by people at Google who do understand this (e.g. Ilya Grigorik), nothing has changed in the last two years:
(Yes, they do allow you to see the distribution but it hardly counts because it's a clumsy histogram to use and since you can't report on it, your boss is going to look at the averages which are displayed prominently everywhere else)
https://developer.mozilla.org/en-US/docs/Navigation_timing
… and the even newer Resource Timing API:
http://www.stevesouders.com/blog/2014/08/21/resource-timing-...
One word of caution: ignore tools like Google Analytics which are designed by the statistically ignorant. If a tool uses the mean or an average rather than a median or n-th percentile, you cannot trust it to make decisions because real-world timing data has staggering outliers – 99.999% of your users may load a resource in less than a second but the two clients which reported 2 hour load times will skew the average so much that you'll think you're seeing a problem which doesn't actually exist.
I wrote about an example of this in 2012 and despite the assurances of people who work on Google Analytics and, I'm told, internal lobbying by people at Google who do understand this (e.g. Ilya Grigorik), nothing has changed in the last two years:
http://chris.improbable.org/2012/5/18/google-analytics-decep...
(Yes, they do allow you to see the distribution but it hardly counts because it's a clumsy histogram to use and since you can't report on it, your boss is going to look at the averages which are displayed prominently everywhere else)