Optimize customizer load time

[Nick_theGeek]

Please see some discussion in slack for background and ideas
​https://wordpress.slack.com/archives/C0381N237/p1559852377000600

TLDR; More plugins and themes are using the customizer. It's a very good experience when making changes to options that affect the display of the site. As more controls are added the initial page render is taking longer and longer.

Searching for things like "WordPress customizer long load times" results in tickets like
​https://wordpress.org/support/topic/customizer-takes-12-seconds-to-load/

In my own tests against a plugin I develop for I've found DOM Ready load times approaching 10 seconds.

In default usage this is not apparent, but as themes and plugins use the customizer it becomes very slow.

Ideally the initial page render should be more complete and then panels, sections, and controls can be added. This will make the page "feel" complete and more responsive.

[celloexpressions]

Does anyone have actionable proposals for how to address this issue?

There are several other open tickets that focus on specific ways that could improve load time. For example by loading customize objects as needed or even re-writing the customizer to be more fully JavaScript-based. I'm not sure if there's value in keeping this ticket open unless it wants to be used as a tracking issue for those other (long term) projects.

[pbearne]

I can't see Cuzomizor getting a rewrite, so can we close this?