It's because of drivers and PnP and especially USB. When you load Win3.1, WinNT4 and lower, drivers load without scanning for hardware presence. It's just a disk to memory copy. In Win95, the first PnP OS, it scans for PnP hardware at every boot. That's slow.

To prove my point, you could try loading some of the USB drivers for DOS or one of the ISA PnP configuration utilities (such as ICU - Intel Configuration Utility), see how fast it boots then!

Also, if you left the network config untouched, it defaults to TCPIP+DHCP, and when DHCP doesn't respond (cable unplugged), it's another 30s delay. Win311 didn't have TCPIP unless you install it manually. It also asks you to configure it during installation - less likely to select DHCP if you don't have it. And then, in Win311, network is started by DOS (NET START in autoexec.bat), not by Windows.