avoiding unnecessary tree state changes #1481

Job	Run time
macos-latest: clang++ Release Threads	35m 23s
macos-latest: clang++ Release OneTBB	3m 39s
macos-latest: clang++ Release PaRSEC	5m 19s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	49m 20s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	2m 54s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	2m 38s
macos-latest: clang++ Debug Threads	26m 36s
macos-latest: clang++ Debug OneTBB	5m 1s
macos-latest: clang++ Debug PaRSEC	4m 14s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	49m 16s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	2m 16s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	3m 11s
	3h 9m 47s

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