It isn't-- because the speedups you conjecture creating already exist.

You propose that your design moves accessing the UTXO data out of the critical path for block validation, on the basis that you will have already validated the transactions and so don't need to access that data.

Bitcoin Core does not have UTXO accesses in the critical path in the same case and doesn't write in the critical path either. So it would not see the benefit you expect for this reason. (not that it wouldn't potentially have benefit, just not the benefit you are suggesting at least not for the reason you suggest)

Cheers