Simplify and fix runahead computation. #6698

Job	Run time
1/4	13m 8s
2/4	13m 25s
3/4	14m 59s
4/4	13m 22s
py-3.11	13m 12s
flaky	6m 24s
_remote_background_indep_poll	5m 17s
_remote_background_indep_tcp	7m 59s
macos 1/5	37m 15s
macos 2/5	45m 2s
codecov	0s
	2h 50m 3s

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