Isn’t a low number of transactions per second essentially built in to the original ETH spec? If so, surely it’s likely that each transaction should be expensive?
Yes, but that's just because a design goal is for modest computers to be able to fully participate in the protocol, and the low number of transactions was the best they could do. They've also been planning to scale since early days. Here are some of Vitalik's blog posts on scaling back in 2014:
And here's one of his 2021 posts, in which he talks about the importance of regular users being able to run a node, but says a million tx/sec should be achievable:
If I remember correctly, L1 ETH can handle ~13 transactions per second. Meanwhile L1 Bitcoin can handle ~4 transactions per second. At the time it was introduced, one of Ethereum's selling points was that it was able to handle 3x more transactions per second than Bitcoin, IIRC.
300% more is usually considered a good upgrade for most things (Apple is touting a whopping 20% speed increase on their latest iPad, for example). It's just that six years is a long time in this space and they've learned how to make things a lot faster since.