It should be easy to test - just cut out two lines with session in database::apply_block_extended (I'm actually assuming that out of order blocks won't reach that routine during sync, but if they do, it would be a source of slowdown).

I'd be surprised if undo sessions were the problem. They are relatively slow and worthy of optimization, but in relation to simple transactions, mostly custom_jsons, so their performance is significant when there is many of them, like during block production, reapplication of pending or in extreme stress tests with colony+queen. During sync we only have one session per block.