Plymouth is not slowing your boot-up process! Plymouth is responsible for the boot-up splash screen. Please read Plymouth.

It loads the boot-up logo at the beginning of the boot-up process and then waits until the boot-up process has finished so it unloads the splash screen. That is all it does and that is why it has to run in parallel and co-exist throughout the whole boot-up process. It does not delay anything, it just waits.

That is exactly what is happening. No more and no less. Please take a look at the output you added to your question and carefully read the following:

● plymouth-quit-wait.service - Hold until boot process finishes up

How to Verify?

You can verify that plymouth-quit-wait.service is not holding anything back but the graphical login screen, by running the command:

systemctl list-dependencies --reverse plymouth-quit-wait.service

which will output all the services that depend on plymouth-quit-wait.service (i.e services that are delayed by plymouth-quit-wait.service). On a freshly installed Ubuntu system, the output will be like this:

plymouth-quit-wait.service
● └─multi-user.target
●   └─graphical.target

Which means only the graphical login screen is configured to wait for the plymouth-quit-wait.service and nothing else.

On the other hand if you list the services that plymouth-quit-wait.service is configured to wait for by running the command:

systemctl list-dependencies plymouth-quit-wait.service

the output would be almost every service that should run on boot and the output will look like this:

plymouth-quit-wait.service
● ├─system.slice
● └─sysinit.target
●   ├─apparmor.service
●   ├─dev-hugepages.mount
●   ├─dev-mqueue.mount
●   ├─grub-initrd-fallback.service
●   ├─keyboard-setup.service
●   ├─kmod-static-nodes.service
●   ├─plymouth-read-write.service
●   ├─plymouth-start.service
●   ├─proc-sys-fs-binfmt_misc.automount
●   ├─setvtrgb.service
●   ├─sys-fs-fuse-connections.mount
●   ├─sys-kernel-config.mount
●   ├─sys-kernel-debug.mount
●   ├─systemd-ask-password-console.path
●   ├─systemd-binfmt.service
●   ├─systemd-hwdb-update.service
●   ├─systemd-journal-flush.service
●   ├─systemd-journald.service
●   ├─systemd-machine-id-commit.service
●   ├─systemd-modules-load.service
●   ├─systemd-random-seed.service
●   ├─systemd-sysctl.service
●   ├─systemd-sysusers.service
●   ├─systemd-timesyncd.service
●   ├─systemd-tmpfiles-setup-dev.service
●   ├─systemd-tmpfiles-setup.service
●   ├─systemd-udev-trigger.service
●   ├─systemd-udevd.service
●   ├─systemd-update-utmp.service
●   ├─cryptsetup.target
●   ├─local-fs.target
●   │ ├─-.mount
●   │ ├─systemd-fsck-root.service
●   │ └─systemd-remount-fs.service
●   └─swap.target
●     └─swapfile.swap

This confirms that plymouth-quit-wait.service is not slowing down anything but just running in parallel waiting for the system to fully boot and then it will hide the boot-up splash screen to bring up the graphical login screen.

To understand more.

Please run the following command in the terminal:

systemd-analyze plot > ~/SystemdAnalyzePlot.svg

Then, look for SystemdAnalyzePlot.svg in your home directory and run it in the image viewer or the Internet browser. You might need to enlarge the image so that you can read the processes names. It is worth checking and will give you a better understanding on how the boot-up process works.

You can, however, reduce your boot-up time by disabling NetworkManager-wait-online.service so plymouth has one less process to wait for. This can indeed reduce your boot-up time. To do this please follow the steps in this answer.

Oh... and please leave plymouth alone, it's not the one that makes you wait...it's the one that waits for you.

I cannot say what else is necessary, but I can maybe give a workaround for you, so that the machine starts faster.

For me the most working solution was to disable the plymouth in grub with

sudo nano /etc/default/grub

and change the line GRUB_CMDLINE_LINUX_DEFAULT into

GRUB_CMDLINE_LINUX_DEFAULT="noplymouth video=SVIDEO-1:d"

After saving the change you must update the grub with

sudo update-grub

and then restart the machine.

You can disable this service by running the command :

sudo systemctl disable plymouth-quit-wait.service

From Service Description:

Hold until boot process finishes up

I was having boot time as 3-4 minutes because of this plymouth service taking 1 minute 30 seconds. What fixed my boot time was updating the kernel from 5.4(my default) to 5.9 in Ubuntu 20.

update your kernel