Performance issue in facets_summary block

Problem/Motivation

#3264284: Facets summary should be cacheable, in case facets are using cacheable source. → added a <code>calculateCacheDependencies() method, it looks like this hits a slow code path loading facets which on the site I'm profiling can take up to 900ms (frequently seeing 4-500ms).

Uploading an xhprof screenshot just to show the issue. Don't have much more information than this at the moment unfortunately.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes