and doubling the capacity solves all scalability problems? That is still minuscule capacity to enable any serious use of bitcoin as means of payment.

And if the simple transaction is me purchasing a pizza instead of transferring a million from one speculator to another? How much does it cost and how much it takes time?

Bitcoin doesn't easily scale in its current form. For that, people have invented the Lightning Network (https://lightning.network). LN is a layer 2 solution which sacrifices a modicum of trustlessness in favor of fewer on-chain transactions. Suppose you want to buy a cup of coffee every day with the same vendor. You could agree to do all transactions in the Lightning Network, reducing the number of on-chain transactions from 30 to 2. This cuts fees and increases usability.