FetchContent has poor performance on Windows

Take this minimal example: https://github.com/iboB/cmake-fetch-content-perf/blob/master/CMakeLists.txt

On Ubuntu 20.04 (more precisely WSL) - CMake 3.16.3 - after the initial slow configure, each subsequent one takes about 200ms per fetch content item. This is not great (and I have a suggestion for improvement, which seems too much for now, in #21698 (closed)), but I guess I can live with 200ms per item.

On Windows - CMake 3.19.2 - however each item takes about 1.2 seconds (the exact same machine). I think this is too slow, and I can find no reason for the 5x slowdown. At first I thought it was because of FetchContent relies on ExternalProject_Add which in turn calls cmake --build and this launches the heavy machinery of MSBuild with my default generator. However I tried it with MinGW Makefiles and saw no difference in performance. Still about 1.2 seconds per item.

Summarized personal measurements from this and below posts:

OS	CMake Ver	Generator	Machine	~ ms per item
Ubuntu 20.04	3.16.3	Unix Makefiles	ThreadRipper, SSD	200
Windows 10	3.19.2	Visual Studio 2019	ThreadRipper, SSD	1200
Windows 10	3.19.2	MinGW Makefiles	ThreadRipper, SSD	1200
Arch Linux	3.19.2	Unix Makefiles	8 core @ 2.4 GHz, HDD	800
Arch Linux	3.19.2	Unix Makefiles	8 core @ 2.4 GHz, RAM	700
Arch Linux	3.16.3	Unix Makefiles	8 core @ 2.4 GHz, HDD	350
Arch Linux	3.16.3	Unix Makefiles	8 core @ 2.4 GHz, RAM	300
Windows 10	3.16.3	MinGW Makefiles	6 core @ 3 GHz, SSD	1500