Network is taking a lot of time at startup

My computer takes about 40 seconds to start (Kubuntu 18.04, without SSD).

This is the output of systemd-analyze critical-chain

~$ systemd-analyze critical-chain
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.

graphical.target @38.283s
└─multi-user.target @38.283s
  └─postfix.service @38.278s +4ms
    └─postfix@-.service @35.039s +3.236s
      └─network-online.target @35.033s
        └─NetworkManager-wait-online.service @26.666s +8.366s
          └─NetworkManager.service @20.984s +5.678s
            └─dbus.service @18.457s
              └─basic.target @18.330s
                └─sockets.target @18.330s
                  └─uuidd.socket @18.330s
                    └─sysinit.target @18.286s
                      └─haveged.service @18.286s
                        └─systemd-tmpfiles-setup.service @16.903s +448ms
                          └─local-fs.target @16.900s
                            └─boot-efi.mount @16.757s +142ms
                              └─systemd-fsck@dev-disk-by\x2duuid-DAA7\x2d28F1.service @14.156s +2.550s
                                └─local-fs-pre.target @13.638s
                                  └─lvm2-monitor.service @4.199s +9.438s
                                    └─lvm2-lvmetad.service @5.365s
                                      └─systemd-journald.socket @4.141s
                                        └─system.slice @4.140s
                                          └─-.slice @4.096s

It looks like NetworkManager-wait-online.service and NetworkManager.service are taking a lot of time to load. Is there any way to reduce their time of oading? Can they be disabled? Will that disable the wifi after startup? I don't always need the wifi to be turned on after startup, I can manually turn it on if needed.

What does NetworkManager-wait-online.service do?