Various small arrow flamegraph perf improvements

[brancz]

Surprising how much more perf improvements we keep finding:

$ benchstat old.txt new.txt
name                old time/op  new time/op  delta
ArrowFlamegraph-10  1.35ms ± 1%  1.15ms ± 1%  -14.25%  (p=0.000 n=10+10)

[alwaysmeticulous]

🤖 Meticulous spotted visual differences in 1 of 400 screens tested: view and approve differences detected.

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