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They explain this further on in the Twitter thread. It's the portion of time spent sorting the array of system calls during boot. This didn't used to matter when it took longer than a second to boot up, but now that the boot times are in the sub-second range, the amount of time spent sorting the array of system calls ended up taking up a notable chunk of the time spent booting.

Might not matter for the things you care about, but some people certainly do care about boot performance.

The author mentions in the thread that the boot time they measured on the device was 28 ms and 7% of it (2 ms) was spent sorting.

On the sorting of SYSINIT (not sysint as the title says) calls during bootup. The actual speedup is only a couple milliseconds but that means the old method was in the order of hundreds of milliseconds. Which is indeed a lot for a sorting operation.

100x faster on 7% of Firecracker boot

So ~5ms speedup