Public-Key servers should accept valid Public-Keys.
And they either cannot refuse to accept massive keys from a single IP, or from zombies, or from a hacked qualified key server via sync requests.
Generating some kinds of public key is quite quick, i.e., Ed25519: high-speed high-security signatures specifies:
Very fast signing. The software takes only 87548 cycles to sign a message. A quad-core 2.4GHz Westmere signs 109000 messages per second.
Fast key generation. Key generation is almost as fast as signing. There is a slight penalty for key generation to obtain a secure random number from the operating system; /dev/urandom under Linux costs about 6000 cycles.
Is it possible attack key servers with massive valid but frivolous Public-Keys continually, so that they cannot continue to work properly?
Either they go offline, or refuse to accept new public keys, or ban certain kinds of key generating algorithms, or delete all keys uploaded in a certain time, or requires registration or CAPTCHA.
And most importantly, how is Public-key Server system designed to be away from this kind of attack?
Edit: In 2014, Werner Koch submitted an I-D for use of Ed25519 in OpenPGP. I have no idea if it is accepted in OpenPGP, but ECC is supported.
As statistics from https://sks-keyservers.net/status/key_development.php says, there are 4.5 Million OpenPGP keys in total, though some sources claims more. Less than 1 thousand keys are added daily, with a maximum exception to be 40 thousand per day.
Since generating Ed25519 public keys is pretty easy, and the amount of public keys is rather small (I had expected it to be trillions in amount and pega-bytes to occupy), to double the keys in one day might not be a joke.
Even if the servers discard keys to avoid garbage outnumber good keys (I wonder if that is the case when negative amount of daily keys added occurs to the statistic), it means some good keys are discarded, too. And you can always perform a distributed attack at any time.
I have no idea if this conclusion applies to all ECC curves. Neither do I have any idea if RSA do, too, by generating weak keys (it doesn't matter even if they are easy to break). But I do wonder how PGP servers survived, as I am rather juvenile in this field.