At the Computer History Museum, I use an IBM 1401 mainframe (1959). When you hit the power button, relays go ch-ch-chunk and it's immediately ready to use. Because it has magnetic core memory, it even has the previous program already in memory, preserved over power-down. Computers have taken many steps backwards as far as startup time. Of course, loading a new program from punch cards is slow, so some things have improved :-)