I think even scp256k1 would be accelerated with some newer instructions (mulx / adcx) if one goes deeper to the asm level. But they are not very widespread instructions yet in terms of use. The sha256 and crc look that they can be sped up quite a bit. As you progress with iguana and do similar profiling work, you'll have to eliminate the bottlenecks one by one and at that point you may hit issues like that.
As for the DB, I'm not sure exactly how the bitcoin core software works. What I do know, is that it's extremely slow with a mechanical disk. I think I've read gmaxwell say that the db is only for the wallet and not for the blockchain. I haven't analyzed it myself (I'm an extremely rusty coder with C aversion, so...) and that's why I can't say anything with certainty.
As for the immutability, I guess one would say that in theory the blockchain is "eventually consistent"... so there's always room for some divergence...