It is talked about in the hashmap docs:

By default, HashMap uses a hashing algorithm selected to provide resistance against HashDoS attacks. The algorithm is randomly seeded, and a reasonable best-effort is made to generate this seed from a high quality, secure source of randomness provided by the host without blocking the program.

The default hashing algorithm is currently SipHash 1-3, though this is subject to change at any point in the future. While its performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings, though those algorithms will typically not protect against attacks such as HashDoS.

Basically, if the attacker has control over the key inserted into a hashmap then with a simple hashing algorithm they can force collisions which results in the hashmap falling back to a much slower linear lookup. This can be enough to stress a server and slow down all requests going through it or even cause it to crash. So a lot of effort is made in the default hasher to mitigate against this. There are faster hashing implementations out there if you are not worried about this that you can opt into. But the default is to be secure.