User expectations for page load are around 2 seconds or faster. The slower a site is, the fewer conversions and return visitors your site will have. Learn how to test, track, and increase the speed of your website.

WordCamp Fayetteville 2015
Michael McCranie

This talk covers topics closely related to those discussed in Baking in the SEO.

Decide what your page load performance goal is (e.g., 2 seconds) and discuss with your clients ahead of time. This way, you can judge feature requests partly by their impact on that metric.

Optimization points include hosting, PHP environment/configuration, theme, database, update status, and images (format selection, sizing, .

Plugins and Tools

WebPagetest Detailed information about web page performance.
Theme Check themecheck.org is based on the Theme Check plugin.
WP-Optimize, WP Sweep There are a number of optimization plugins you can run after making large changes to your database (e.g., optimize database after deleting revisions). Be sure to backup your entire site before optimizing.
EWWW Image Optimizer, Imsanity, Hammy Alternatives to WP Smush.
WP Rocket, W3 Total Cache, WP Super Cache Caching and minification alternatives can be a pain to get working (although once they do, they work well). WP Rocket is a paid service that just works.