What problem are you trying to solve?

If you just want constant-time operations, you just need dedicated instructions. Maybe they run on dedicated cores, but that's an implementation detail. I think this is a good idea, and it feels like we're heading this way already, more or less.

If you want to address all of the vulnerabilities that have arisen, you need full isolation. As mentioned in a sibling comment, you can't share any memory or caches etc. Each "security core" would have to be fully isolated from every other core, including every other security core. And you also need to segregate out "sensitive" code and data from non-sensitive code and data. When all is said and done, I don't see how we're talking about anything less than scrapping all existing ISAs, and perhaps even von Neumann architecture itself. I'm sure there are some middle-ground solutions between where we are today and this extreme, but I think they're going to look more like TPM/Secure Enclave than special cores, and they're only going to partially address the vulnerabilities.