Use systemd-analyze blame:

➜  ~ systemd-analyze blame
          3.475s plymouth-quit-wait.service
          2.347s postgresql@9.6-main.service
          1.495s dev-sda5.device
          1.279s systemd-journal-flush.service
          1.086s apparmor.service
          1.028s plymouth-read-write.service
           949ms snapd.service
           897ms fwupd.service
           532ms dev-loop17.device
           523ms dev-loop14.device
           517ms dev-loop13.device
           511ms dev-loop16.device
           502ms dev-loop18.device
           (...)

Or have it presented on a plot: Systemd-analyze plot > boot.svg