As per my knowledge When you increased the block creation time (BCT), it slowed down how often new blocks were made. With longer gaps between blocks, there might not have been enough transactions to fill them up, leading to empty blocks.
Increasing the block size (BS) helps fit more transactions in a block. When the block size is big and the network is busy, it's easier to fill blocks with transactions, reducing empty ones.
So, in your case, the slower block creation time (BCT 10 sec) and bigger block size (BS 32MB) could have caused more empty blocks because transactions didn't arrive quickly enough to fill the extended gaps between blocks.
Salesforce Marketing Cloud Training