I've seen someone say that a "power throttling" feature in Windows 10 is causing this. Here's a blog post about that feature:
That post says that using the high performance power plan is not enough anymore:
Previously, you could disable the Power Throttling feature by enabling the High Performance power plan in Windows 10. In Fall Creators Update (Windows 10 version 1709), there is a dedicated Group Policy option which can be used to disable Power Throttling. Here is how.
Perhaps that's the important change causing those LatencyMon results in 1903 compared to previous builds?
I'm wondering if that extra latency is perhaps intended. Maybe this is how the Windows kernel actually throttles background programs, by making the programs wait an extra long time when they ask for something, and LatencyMon then sees that a spike.