The underlying implementation of contract storage is a Merkle Patrica Tree, which is expensive for read/write. Further, if you write a new entry into the tree (i.e., increasing balance from 0 to non-zero), EVM will charge 20,000 gas in EIP2200. The cost can be even higher for _transferFrom, which is widely used by contract interactions with extra write to allowance map.
Since the MPT access cost is so high, we are proposing an upgraded ETH without MPT, which could achieve at least 4-5x performance increase based on our experiments.