Why 13 TPS? Where does the time go?

(I don't mean this to be a Blockchain Holy War question! I'm asking a technical question, not a philosophical one.)

I keep hearing that Ethereum layer-1 is capable of doing something on the order of 13 transactions per second (give or take, depending on the complexity of the transaction). Why is that? Like, what exactly is so expensive about processing Ethereum transactions, such that a normal computer can only do 13 of them in a second?

I understand that L1 (rightly!) prioritizes decentralization, and that from now on apps should be built on L2s, and that L2s' TPS can be much much higher.

I understand that Ethereum L1 is meant to run on consumer hardware, for the sake of decentralization, and so throughput is intentionally kept low enough that reasonably-cheap hardware can handle it.

I'm just kinda confused because 13 TPS still seems like a surprisingly low number for a modern-but-ordinary consumer laptop. How is all that time being spent?

(e.g. I'm imagining that the answer might be something like, "Modern cryptographic hash functions are pretty expensive to run, and Ethereum nodes have to do a whole bunch of hashes for every block, and that's where most of the time goes." Or, "Every block requires downloading a whole bunch of data; most of the time is spent waiting for data to be transferred over the network." Or something like that. But I'm just making those up; I don't know what the real costs are, so I'm wondering if anyone here can help me understand.)