Most importantly GPU drivers. Run dxdiag.exe, Display tab, it should say DirectDraw and Direct3D acceleration are both Enabled. For that particular iMac you need two of these drivers, because two GPUs.
Another possible thing is SATA driver. Normally installed automatically as a part of Windows, but I never tried running windows on Apple desktops. If Apple gives you Windows drivers install them, otherwise install CPU-Z, “Mainboard” tab, see the chipset, then install from intel.com.
I couldn't wait for this evening so poked around during lunch. dxdiag was fine, but the SATA driver was a generic AHCI SATA driver. Replacing that with a chipset-specific SATA driver dramatically improved the responsiveness of Explorer and general application launching.
I can't be 100% sure that solved everything, but it is perceptibly faster now. I'm not sure why Windows didn't auto-install those drivers. Perhaps it would have eventually done so during one of the Windows Update cycles, but your advice was very helpful, so thank you!
I have experienced that only once.
The computer was a $150 tablet from ~10 years ago, with 1GB RAM, 32 bit Windows 10 (sold with Windows 8, I think), and Intel Atom SoC for a CPU.
Computers with adequate amount of resources worked well for me, even very old ones.