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Just like other errors, it is easy to miss out on asset optimisations if you don't have tests or monitors for them.

Monitoring solutions like New Relic provide you high level insights on scopes for further optimizations. You may be particularly interested in their Data transmission analysis and load timing analysis with time segmentation.

Yslow is another free and very popular solution which analyses pages and grades them based on Yahoo!'s rules for high performance web sites.

For larger sites with hundreds of pages, if you really need to squeeze out performance, you will have to augment your integration tests and acceptance tests with automated performance threshold verification. One of the best ways to do so is through HAR log analysis.

Most modern test automation solutions for web applications (including PhantomJS and Selenium through BrowserMob proxy) provide a way to generate HTTP Archive logs (HAR logs) which contain complete information about the browser's interaction with the server over time - including requests made, time taken to serve them, their sizes, and optionally the entire content of each request body.

From wikipedia:

The HTTP Archive format or HAR, is a JSON-formatted archive file format for logging of a web browser's interaction with a site. The common extension for these files is .har.

The specification for the HTTP Archive (HAR) format defines an archival format for HTTP transactions that can be used by a web browser to export detailed performance data about web pages it loads. The specification for this format is produced by the Web Performance Working Group[1] of the World Wide Web Consortium (W3C). The specification is in draft form and is a work in progress.

Chrome network tab has a "Save as HAR with content" option which is very helpful when writing the test cases and for manual inspection. For visualizing saved archives, this chrome extension comes handy.