What is a cold cache?

A cold cache is one that holds no usable copy of the pages being requested, so each request is a miss that goes to the origin. A CDN cache goes cold after a purge or a deploy, in a PoP that has not served a page recently, and for any new URL or variant.

after purge:  cf-cache-status: MISS   (origin round trip)
next request: cf-cache-status: HIT

The first visitors after a purge pay for it:

• Slower responses, because the TTFB includes the origin round trip and the time to generate the page.
• A burst of origin load as many pages miss at once, softened per location by request collapsing.
• A lower hit ratio until traffic, or a warmer, fills the cache again.
• Crawlers that arrive during that window also meet slower responses, as covered under crawl budget.

A cache warms up by itself as visitors request pages, but slowly for pages with little traffic per region. Cache warming does it on purpose, right after the purge.

How long a cold start lasts depends on each CDN's tiers and purge options; the Top 10 CDNs comparison compares ten of them.