The current network hashrate is around 150 EH/s, which means you need to compute around 90,000,000,000,000,000,000,000 hashes for every block. This is an incredible amount of computation with energy costs comparable to smaller countries.
Creating a merkle tree from mempool transactions and validating them, on the other hand, can be done easily even on a low-end consumer laptop. Now, there are multiple miners and mining pools doing that, but it doesn’t change the overall picture. Proof of work easily dominates all other computation done in the bitcoin network.