libcurl caches different information in order to help subsequent transfers to perform faster. There are three key caches: DNS, connections and TLS sessions.
When the multi interface is used, these caches are by default shared among all the easy handles that are added to that single multi handle, and when the easy interface is used they are kept within that handle.
You can instruct libcurl to share some of the caches with the share interface.
When libcurl resolves a host name to one or more IP addresses, that is stored in the DNS cache so that subsequent transfers in the near term will not have to redo the same resolve again. A name resolve can easily take several hundred milliseconds and sometimes even much longer.
By default, each such host name is stored in the cache for 60 seconds (changeable with CURLOPT_DNS_CACHE_TIMEOUT).
libcurl does in fact not usually know what the TTL (Time To Live) value is for DNS entries, as that is generally not exposed in the system function calls it uses for this purpose, so increasing this value come with a risk that libcurl keeps using stale addresses longer periods than necessary.
Also sometimes referred to as the connection pool. This is a collection of previously used connections that instead of being closed after use, are kept around alive so that subsequent transfers that are targeting the same host name and have several other checks also matching, can use them instead of creating a new connection.
A reused connection usually saves having to a DNS lookup, setting up a TCP connection, do a TLS handshake and more.
Connections are only reused if the name is identical. Even if two different host names resolve to the same IP addresses, they will still always use two separate connections with libcurl.
Since the connection reuse is based on the host name and the DNS resolve phase is entirely skipped when a connection is reused for a transfer, libcurl will not know the current state of the host name in DNS as it can in fact change IP over time while the connection might survive and continue to get reused over the original IP address.
The size of the connection cache - the number of live connections to keep there - can be set with CURLOPT_MAXCONNECTS (default is 5) for easy handles and CURLMOPT_MAXCONNECTS for multi handles. The default size for multi handles is 4 times the number of easy handles added.
TLS session cache
TLS session IDs and tickets are special TLS mechanisms that a client can pass to a server to shortcut subsequent TLS handshakes to a server it previously established a connection to.
libcurl caches session IDs and tickets associated with host names and port numbers, so if a subsequent connection attempt is made to a host for which libcurl has a cached ID or ticket, using that can greatly decrease the TLS handshake process and therefore the time needed until completion.