That's interesting. A project at work is affected by Windows slow open() calls (wrt to Linux/Mac) but we haven't found a strong solution rather than "avoid open() as much as you can".
It's likely Windows Defender, which blocks on read I/O to scan files. Verify by adding a folder to its exclusion list, though this isn't helpful if it's a desktop app. The difference is most noticeable when you're reading many small files.