Optimizing kernel startup times

[t42]

...disconnect individual USB ports one by one. Considering ambiguous recovery I'd try kernel of another series, for LM19 there is an older one 4.15 in Grub advanced menu or to install newer 5.11.16 from Ubuntu mainline.

[caranthir]

Here are my testing results so far:

Unplug headset USB only

Startup finished in 13.385s (kernel) + 2.679s (userspace) = 16.064s
graphical.target reached after 2.672s in userspace

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Unplug mouse only

Startup finished in 13.221s (kernel) + 2.845s (userspace) = 16.067s
graphical.target reached after 2.766s in userspace

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Unplug keyboard only

Startup finished in 12.289s (kernel) + 2.723s (userspace) = 15.013s
graphical.target reached after 2.717s in userspace

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Unplug all

Startup finished in 6.374s (kernel) + 2.931s (userspace) = 9.305s
graphical.target reached after 2.924s in userspace

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Switch temporarily to 4.15 kernel
Startup finished in 5.974s (kernel) + 14.377s (userspace) = 20.352s
graphical.target reached after 14.371s in userspace

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So, this raises a question...
Why is it all-or-nothing (I have to unplug everything and the boot is much faster; it's apparently not tied to any one device)?

[t42]

The systemd-analyze results probably should be confronted with time intervals measured with real stopwatch, for example from grub menu to the login prompt. It may be that the difference will be not of practical significance.