I have located a Visual Studio binary distro for Windows, and will install and try it. Although if there truly is a bug in the MinGW Windows binary distro, I would have thought it would be well-known.
Visual Studio binary distro for Windows appears to work properly. The program starts up almost instantly; there is no 30-second wait for the command prompt. It loads the 2500x2500 12MB matrix text file in 20 seconds. Still way too slow, but a whole lot better than 90
seconds. Note: this is the one that uses VS runtime libraries. The first one I installed and tried apparently does not.