With the network hash rate so extraordinarily high, why is the transaction capacity so low? Wouldn't it be great if we could focus all that hash power on faster transaction processing instead of a high block hash difficulty? Well, we can!
The best blockchain is the longest chain with the greatest POW, therefore, for any given network hash rate, blockchain length and block difficulty are interchangeable. Transaction capacity per second is the average transactions per block divided by the average block interval in seconds. To increase transaction capacity, the block interval can be reduced by reducing the block hash difficulty. The protocol tightly regulates the block hash difficulty to maintain a predictable rate of inflation, however, if the block reward is reduced from the norm by the same proportion as the block hash difficulty, it will increase transaction capacity and maintain the target rate of inflation, all with the same level of security.
In short, Bitcoin can be faster than VISA if it needs to be.
Transaction capacity and hash rate have nothing to do with each other. As an aside, this is why all the articles/people saying "Bitcoin uses so-and-so much energy per transaction" is nonsensical because hash rate (or power usage) is in no way connected to transaction capacity.
Transaction capacity has to do with the amount of data sent to and stored on the blockchain, not the number of hashes processed by mining machines. Two completely different systems in Bitcoin.
Only way to increase transactions is to increase the amount of data being processed by, and stored to, the Bitcoin network.
There are two levers to change this: the block time and the block limit. But increasing the data sent to Bitcoin would at some point affect the operation of the network by creating many more orphaned blocks as well as hurting decentralization by making it harder and more expensive to run nodes. And Bitcoin's utility is essentially entirely based around its decentralization. That's where all its revolutionary features come from and that is a lot of what separates it from every other cryptocurrency out there. Crypto chose to abandon what makes Bitcoin strong in order to have more transactions. Bitcoin stayed unique and irreplaceable by being the first and only cryptocurrency totally committed to be really f**king good money and decentralization is an absolute requirement for that.
Just look at Ethereum. It has to do much faster transactions and more transactions and has to handle much more data due to smart contracts. And the operation of its nodes are very centralized, plus now with PoS its transaction validators are very centralized as well. A lot of the Ethereum network runs off AWS because it is too much of a hassle to run your own nodes, and there are not a whole lot of full archival nodes that the network relies on because the requirement are too high for the average Ethereum crypto super user.
Now could Bitcoin change one or both of those levers a bit (block time and block limit) to increase transactions? Sure. It's not like changing it a bit is going to suddenly destroy Bitcoin's decentralization. It'd be weakening Bitcoin's decentralization on a sliding scale, so if you didn't change the numbers too much Bitcoin would still be extremely decentralized, just a bit less so. But that would require a hard fork and one of the main facts that makes Bitcoin extremely good money is that it is immutable: it doesn't hard fork. This was the debate in 2017 that led to little sections of the community breaking off to create the sh*tcoin BCH, which then further broke into BCH and BSV, both of which are completely irrelevant. Meanwhile Bitcoin soft-forked with Segwit and remained immutable.
And what would be the upside to hard forking Bitcoin to get more transactions? What maybe you like 4x the number of transactions. Okay great then Bitcoin can handle like 20 tx/s which is still nothing compared to secondary networks in the finance world so you're still left with a Bitcoin network that can't handle much on the base layer, but you've broken Bitcoin's promise of immutability and made the whole network less decentralized and therefore weakened every use case for Bitcoin.
All payment systems have a secure slower base layer and then build more efficient networks on top. You compare Bitcoin to VISA. VISA is not a base layer network for fiat transactions. The base layer is like Fedwire or something and it is much much slower. VISA is a secondary network. So if you want to compare Bitcoin to VISA you have to make the comparison with the LN. LN is, so far, the main network built on top of bitcoin for more efficient payments. It's VISA vs Bitcoin LN, not VISA vs Bitcoin. And LN is both much cheaper and much faster than VISA, though granted it still needs a lot of work to be ready for mass usage.