The speed ratio is much smaller than you say with memory-hard PoW problems, which depend on the amount of RAM you have (and its response time). But it's surely true that a spammer could create many accounts per day, perhaps 1000 per hour on a big server, which could then go on to spam a few accounts each before becoming uneconomical to keep using.

But that would still put the CPM of the spam around US$2, which very few spammers can afford. Maybe mesothelioma lawyers and spearphishers.

You don't have to make spamming physically impossible, just unprofitable.