Technically there’s more than counting, processing runs in parallel, and it’s hard to isolate a bottleneck.
Channel Pipeline says what goes on inside . About → Show → Log → Live → Verbose shows file work, however for easier large scale use (and timing to the second), it’s worth setting up a log-file maybe with log-file-log-level=verbose to see if time per file times 150 million is ballpark reasonable for waiting days.
I have to say I worry about checking that many files over NFS which Duplicati obtains access to via OS.
snapshot-policy is probably the fastest way to identify changed files, but it requires LVM to be available, and TrueNAS may or may not be willing. But even without that, I’d think a local file scan would be faster.