Not every transaction depends on the transaction before it. You can parallelize 99.9% of transaction processing and keep 0.01% serial [if one transaction depends on the one before it in the same block].
What is the source of your numbers?
I don't have a source. I don't think it's needed in this case.
Look at this block for example:
https://blockchain.info/block/000000000000000000c0e693dd6c552e8ba40ac63d978f7cef1b49cce03f8c7dWhile there are some chains of transactions [I can see even one chain of 10tx!] they are exception. Most of transactions are using transactions from previous blocks. These can be easily parallelized.
Besides, even if you had 4 long chains of 300tx in one block, it'd max out 4 core cpu to 100% allowing for about the same time.