Miners can mutate nonce (32 bits) + time (mutates once a second). This allows for 232 (~4 billion) hashes per second. That's not enough anymore for our ASICs as they perform in the TH/s now rather than GH/s. So we allowed miners to mutate the coinbase transaction, but this requires us to generate a new Merkle tree. This means that a miner needs to generate a new Merkle tree every 232 hashes. at 1 TH/s The miner must generate a new Merkle tree 250 times per second.
TLDR: Is Bitcoin PoW actually SHA256 + Merkle tree generation? And not pure SHA256?
If I'm correct in asserting that Bitcoin PoW is SHA256 + Merkle tree, does this slow the commoditization of ASICs and therefore slow decentralization, as ASICs now must be more complex than if they did with just SHA256 + nonce mutations?