How would a proof-of-work system, let alone a blockchain, work? Apparently, GnuPG breaks badly at 150 000 signatures. You want adding a signature to be doable

How would a proof-of-work system, let alone a blockchain, work? Apparently, GnuPG breaks badly at 150 000 signatures. You want adding a signature to be doable on a really old laptop and/or low-end Android phone; a motivated attacker can just choose to expand 1000 000 times as much effort as a not-too-interested user on antiquated hardware. Of course you can make adding a signature more expensive the more signatures are already there, but that lets a motivated attacker make it impossible to vouch for certain users (keys.) Etc.