ntpsec unsets MAKEOPTS. I have repeatedly built ntpsec without this in tmpfs on a 12-core cpu and have not been able to produce a parallel-build failure.
git blame fingers this commit in the -9999 ebuild as the place where it was first introduced:
Maybe it was a transient failure fixed in later git versions?
Difference in build times on Ryzen 9 3900X:
IMO not an NTPsec issue. NTPsec does not respect GNU autoscrap MAKEOPTS. Kick it over to the waf eclass maintainer(s) and ask them to handle it, or write and become the maintenance slave for autoscrap build if you are so inclined.