Does hiring more developers to a team speed up development?

My friend and I are working on an mobile app. We have an ios developer and a backend developer. we are trying to integrate socket as well as make alot of UI changes and we’ve been told that it will take about 2 to 3 months to integrate socket, and about 4 months for all of our updates to be complete. This is too long for us, and I suggested that we hire another developer to speed the process up. However, I was told that its common knowledge that this doesnt speed it up at all, like how you cant hire someone to speed up a pregnancy. I have a very hard time believing this, and am hoping for some professional opinions. Thanks!