During incubation we're still getting incubatingObjectCountChanged at which point we'd also call incubate again, leading to tens of incubate calls nested within each other leading to blockage and significant slowdowns.
This saves one second(!) of Plasma startup time for me, bringing it down from 3.5s to 2.5s for one 4K screen from plasmashell start till panel is up, measured scientifically accurate with a stopwatch.
This also cuts first time opening of Plasma popups in half. Calendar shows up significantly quicker, previously taking over a second, now to just under half a second. Kickoff also takes 300ms instead of 600ms on first invocation for me.