I think there are several points of why Bitcoin is still slow.

In principle, its architecture with which it was designed in its time was in accordance with the needs (performance) of those times, I think they did not anticipate the success it would have in the future and in our days.

Some parameters of its slowness could be:

1.- The type of consensus to decide which miner will win to carry out the transaction by its nature is slow depending on the potential that it needs at the computational level (solve the mathematical puzzle that is this).

2.- The verification to accept a transaction is another reason (merkle tree) how each transaction is classified to prevent double expenses.

Among others.

However, I believe that due to its nature of being the first digital currency that this financial system proposed, it will always be in demand.