Those lags are exactly what we experienced. Click on start menu, get a blue spinner. Folder in explorer? Same. Click on the settings panel, get a spinner. And not for 100ms, but for a second or more. Now, this was on a fresh install, so maybe some cache needed to be built, but it was maddening.
We did it in part to benchmark some games which I do suspect will work better in Windows.
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!
Even on an ancient computer once I turn off menu and window animation the start menu and small folders open with 0 spinning and probably less than 20ms.
Maybe your “fresh install” has a bunch of bloatware and virus scanners?
Son is trying to do gaming in Minecraft with consistent 60+ fps and screen recording in OBS. (The combination of those two was hard to achieve on the Mac and so we're trying Windows as the base OS. The Mac would do 120+ fps until launching OBS, then would struggle to do better than 30 fps.)
We did it in part to benchmark some games which I do suspect will work better in Windows.