Found during recent benchmarking
On relatively weak SSD, with usual (on Windows), non-buffered IO, I noticed that from 20-60MB/s writes on the disk during the benchmark, about 7MB/s goes into file D:\$Log (NTFS).
Apparently, file metadata changes are stored there, such as file update times.
Using NtFlushBuffersFileEx with FLUSH_FLAGS_FILE_DATA_SYNC_ONLY improves performance significantly (from 11-13K qps to 15-16 qps), and D:\$Log writes are now gone.
I conclude we should switch to using this function on Windows.