我的 Ubuntu 20.04 启动非常慢（我必须等待 43 秒才能出现加载程序动画，并且 3 分 5 秒后才到达登录屏幕），当我按下 ESC 时，我可以看到是什么导致启动缓慢：
$ systemd-analyze critical-chain
The time when unit became active or started is printed after the "@" character.
The time the unit took to start is printed after the "+" character.
graphical.target @2min 56.581s
└─multi-user.target @2min 56.581s
└─snapd.seeded.service @2min 18.804s +4.297s
└─snapd.service @1min 41.918s +36.884s
└─basic.target @1min 41.661s
└─sockets.target @1min 41.661s
└─snapd.socket @1min 41.660s +1ms
└─sysinit.target @1min 41.569s
└─snapd.apparmor.service @1min 38.168s +3.400s
└─apparmor.service @1min 35.428s +2.739s
└─systemd-journald.socket @5.225s
└─-.mount @5.214s
└─system.slice @5.214s
└─-.slice @5.214s
$ systemd-analyze blame
1min 4.772s plymouth-quit-wait.service
36.884s snapd.service
28.212s dev-sda5.device
23.701s systemd-journal-flush.service
19.714s dev-loop42.device
19.475s dev-loop39.device
19.231s dev-loop43.device
19.156s dev-loop44.device
19.067s dev-loop31.device
18.919s dev-loop30.device
18.899s dev-loop45.device
18.664s dev-loop34.device
18.634s networkd-dispatcher.service
18.286s vboxdrv.service
18.196s dev-loop41.device
17.673s dev-loop40.device
17.534s dev-loop25.device
17.533s dev-loop38.device
17.261s dev-loop37.device
17.103s dev-loop21.device
17.060s dev-loop36.device
17.060s dev-loop28.device
17.054s dev-loop35.device
16.123s dev-loop11.device
15.913s dev-loop24.device
15.800s dev-loop29.device
15.796s dev-loop33.device
15.242s dev-loop32.device
15.206s dev-loop26.device
15.204s dev-loop22.device
14.790s dev-loop23.device
14.670s dev-loop9.device
14.484s dev-loop13.device
13.823s dev-loop20.device
13.569s dev-loop16.device
13.400s udisks2.service
13.268s dev-loop14.device
12.721s dev-loop27.device
12.686s accounts-daemon.service
12.534s dev-loop18.device
12.303s dev-loop19.device
11.989s dev-loop12.device
11.731s dev-loop15.device
11.353s dev-loop17.device
11.224s dev-loop10.device
10.506s ModemManager.service
10.371s NetworkManager-wait-online.service
10.299s dev-loop2.device
10.285s dev-loop7.device
10.274s dev-loop5.device
9.945s dev-loop8.device
9.771s dev-loop1.device
9.433s NetworkManager.service
9.180s dev-loop3.device
9.108s dev-loop4.device
9.108s dev-loop6.device
8.705s polkit.service
7.710s avahi-daemon.service