[em_te]

Transaction time was arbitrarily set to allow time for a majority of parties on the Internet to verify transactions. It is a trade off of speed versus security. It was picked as a common denominator so that people on the opposite side of the globe and on slightly slower connections can participate in the verification process. That's why changing to PoS doesn't affect it.

Network capacity isn't affected because the amount of data exchanged is the same.

Gas prices isn't affected because gas price is only correlated with the transactions per second (which isn't changed as transaction time isn't changed) and demand and supply for transactions. When lots of people need to make transactions, demand goes up and price goes up. Vice versa. This is the same with other chains too.

Some chains intentionally lower the transaction time to increase throughput to achieve a lower gas fee, but the disadvantage is that only fast peers can verify the transactions fast enough which leads to centralization.