If I give a $100 to you, a centralized entity like my bank just has to subtract 100 from a database which contains my my account balance and 100 to your account balance. And now the transaction is complete. In the case of bitcoin, you still have to do this, but you also need to run a proof-of-work competition among a whole bunch of miners. This proof-of-work competition has to cost significant money in comparison to the amounts being transacted on the network, because if it didn't, a 51% attack would become easy.
Suppose for the sake of argument that the cost of a bitcoin transaction, by which I mean the money invested in mining equipment and cost of electricity of the the mining network, is $1. Assume for simplicity that there's only one transaction per block. Now suppose I'm a malicious actor who just paid you $1000 through the bitcoin network and would now like to execute a double-spend attack. Amassing more hashpower than the rest of the network and "overtaking" the longest chain after 6 confirmations is, on average, only going to cost me 7 * $1.01. In other words, this network with a cost per transaction of $1 is insecure for transacting amounts like $1000. The point is, the mining rewards + transaction fees on the btc network, which in turn are equal or slightly greater than the the cost per transaction block, have to be roughly similar to the amounts being transacted on the network for it to be secure.
The TLDR is this: for a secure btc-like network, transaction cost ~ O(transaction amount). For a centralized ledger (bank): transaction cost ~ O(1). By no means can you call this "efficient."
I'm not saying bitcoin is useless. It is a very clever trustless distributed ledger, but this trustlessness comes at a significant cost.