I've just read the following question Does block size effect Miner's Hashing speed?
and it appears that the answer is no - since miners only need to hash the Merkle root.
However, updating the Merkle root will take a lot of CPU power if/when bitcoin transactions reach a rate of 4000 tps.
What is the incentive for miners/pools to expend effort maintaining the merkle root at this rate? Obviously if they do not expend this effort the result will be an ever growing backlog of unverified transactions.