Wow, this article is great (and crazy long, kind of speaks to how complicated things have gotten): https://www.smashingmagazine.com/2020/01/front-end-performance-checklist-2020-pdf-pages/

Chris Coyier with a great post that sums up some easy front-end performance enhancements http://mediatemple.net/blog/tips/low-hanging-fruit-web-performance/  

I’ve always worked hard to optimize all my web apps (client caching, gzip compression, image compression, domain shading, css sprites, etc…). It’s great to see that it really does make a difference. wpostats.com shows the real stats and puts into hard numbers that perfomance does matter.

Spending time on improving page load time and app interaction time is super important. How important? Check these stats here: http://www.guypo.com/17-statistics-to-sell-web-performance-optimization/