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> How can they efficiently do that?

The web server sends a Signed Certificate Timestamp in the TLS Handshake¹. The browser will check that.

Apache support is coming², and other web server vendors are probably working on it as well.

¹https://tools.ietf.org/html/rfc6962#page-13

²https://httpd.apache.org/docs/trunk/mod/mod_ssl_ct.html

Even better is embedding the SCTs in the x509 structure itself so that you don't have to rely on obtaining/caching and the sending in the handshake. (Yes, there's some cases where a policy change my require the addition of additional SCTs—or different ones altogether—but this should be the exception not the norm.)

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